MFC r309412,r316109,r316132:

r309412 (by imp):

dd is currently a bootstrap tool. It really doesn't have any business
being a bootstrap tool. However, for reproducible build output,
FreeBSD added dd status=none because it was otherwise difficult to
suppress the status information, but retain any errors that might
happen. There's no real reason that dd has to be a build tool, other
than we use status=none unconditional. Remove dd from a bootstrap tool
entirely by only using status=none when available. This may also help
efforts to build the system on non-FreeBSD hosts as well.

r316109:

Don't hardcode input files for stage 1/2 bootloaders; use .ALLSRC instead

This is a better pattern to follow when creating the bootloaders and doing
the relevant space checks to make sure that the sizes aren't exceeded (and
thus, copy-pasting is a bit less error prone).

r316132:

Parameterize out 7680 (15 * 512) as BOOT2SIZE, similar to sys/boot/i386/zfsboot/...

This is being done to make it easier to change in the future--this action might be
needed sooner rather than later because of gcc 6.3.0 bailing, stating that there
is negative free space left (deficit) in the boot2 bootloader.

MFC 278582:
MFi386:
When building some of the boot loaders with clang, and DEBUG_FLAGS or
CFLAGS having '-g' in it, clang outputs several assembly directives that
are too new for our version of binutils.

Therefore, assemble the resulting .s files with clang instead. A more
general solution can be implemented when a GNU as-compatible driver for
clang's integrated assembler appears.

MFC r280980:

Ensure the cross assembler, linker and objcopy are used for the build32
stage, just like for the regular world stage.

Reviewed by: rodrigc, imp, bapt, emaste
Differential Revision: https://reviews.freebsd.org/D2187

MFC r279598:

When compiling boot2 with gcc on i386 and pc98, only use the custom flag
-mno-align-long-strings when compiling with base gcc. This is checked
by comparing the version number against 4.2.1, which is not exactly
right, but good enough. (There is no other way to check whether we are
using the non-standard gcc in base, as far as I know.)

Reported by: rodrigc

MFC: r268474 and r268476 (by imp)
- Merge the clang support from i386.
- Compile boot2 with clang on pc98.

MFC: r272250 (by nyan)
- Reduce diffs against i386.

MFC r260334:

Split the last gcc-specific flags off into CFLAGS.gcc. This also
removes the need to use -Qunused-arguments for clang throughout the
tree.

MFC r260369:

Apply band-aid for 32-bit compat libs failures after r260334: put back
-Qunused-arguments for clang for now, until I can figure out a way to
make it unneeded in all scenarios. Sorry about the breakage.

MFC r260095:

For sys/boot/i386 and sys/boot/pc98, separate flags to be passed
directly to the linker (LD_FLAGS) from flags passed indirectly, via the
compiler driver (LDFLAGS).

This is because several Makefiles under sys/boot/i386 and sys/boot/pc98
use ${LD} directly to link, and the normal LDFLAGS value should not be
used in these cases.

MFC r257532 (by adrian):

Fix this build for clang.

MFC r259730:

To avoid having to explicitly test COMPILER_TYPE for setting
clang-specific or gcc-specific flags, introduce the following new
variables for use in Makefiles:

CFLAGS.clang
CFLAGS.gcc
CXXFLAGS.clang
CXXFLAGS.gcc

In bsd.sys.mk, these get appended to the regular CFLAGS or CXXFLAGS for
the right compiler.

MFC r259913:

For libstand and sys/boot, split off gcc-only flags into CFLAGS.gcc.

MFC r259927:

Fix pc98 build, by also forcing COMPILER_TYPE in sys/boot/pc98/boot2's
Makefile.

Pointy hat to: dim

MFC r280980:

Ensure the cross assembler, linker and objcopy are used for the build32
stage, just like for the regular world stage.

Reviewed by: rodrigc, imp, bapt, emaste
Differential Revision: https://reviews.freebsd.org/D2187

MFC r279598:

When compiling boot2 with gcc on i386 and pc98, only use the custom flag
-mno-align-long-strings when compiling with base gcc. This is checked
by comparing the version number against 4.2.1, which is not exactly
right, but good enough. (There is no other way to check whether we are
using the non-standard gcc in base, as far as I know.)

Reported by: rodrigc

MFC: r268474 and r268476 (by imp)
- Merge the clang support from i386.
- Compile boot2 with clang on pc98.

MFC: r272250 (by nyan)
- Reduce diffs against i386.

MFC r260334:

Split the last gcc-specific flags off into CFLAGS.gcc. This also
removes the need to use -Qunused-arguments for clang throughout the
tree.

MFC r260369:

Apply band-aid for 32-bit compat libs failures after r260334: put back
-Qunused-arguments for clang for now, until I can figure out a way to
make it unneeded in all scenarios. Sorry about the breakage.

MFC r260095:

For sys/boot/i386 and sys/boot/pc98, separate flags to be passed
directly to the linker (LD_FLAGS) from flags passed indirectly, via the
compiler driver (LDFLAGS).

This is because several Makefiles under sys/boot/i386 and sys/boot/pc98
use ${LD} directly to link, and the normal LDFLAGS value should not be
used in these cases.

MFC r257532 (by adrian):

Fix this build for clang.

MFC r259730:

To avoid having to explicitly test COMPILER_TYPE for setting
clang-specific or gcc-specific flags, introduce the following new
variables for use in Makefiles:

CFLAGS.clang
CFLAGS.gcc
CXXFLAGS.clang
CXXFLAGS.gcc

In bsd.sys.mk, these get appended to the regular CFLAGS or CXXFLAGS for
the right compiler.

MFC r259913:

For libstand and sys/boot, split off gcc-only flags into CFLAGS.gcc.

MFC r259927:

Fix pc98 build, by also forcing COMPILER_TYPE in sys/boot/pc98/boot2's
Makefile.

Pointy hat to: dim