Build lib/msun tests with compiler builtins disabled

This forces the compiler to emit calls to libm functions, instead of
possibly substituting pre-calculated results at compile time, which
should help to actually test those functions.

Reviewed by: emaste, arichardson, ngie
Differential Revision: https://reviews.freebsd.org/D28577

(cherry picked from commit cf97d2a1dab8f2cddc4466fe64d37818339c73be)

riscv: Add a soft-float implementation of fabs()

We could just use a C implementation using __builtin_fabs(), but using
this assembly version guarantees that there is no additional prolog/epilog
code. Additionally, clang generates worse code for masking off the top bit
than GCC: https://bugs.llvm.org/show_bug.cgi?id=49377.

This fixes the RISCV64 softfloat world build after cf97d2a1dab8. That commit
added -fno-builtin to the msun tests which resulted in the first references to
fabs (previously the compiler inlined all calls).

Reviewed By: dim
Reported by: mjg
Differential Revision: https://reviews.freebsd.org/D28994

(cherry picked from commit 524b018d200408bed5eb0d2b892db5b9fb46808b)

riscv: Fix whitespace issues in fabs added in 524b018d2004

(cherry picked from commit 066dab17e7a4a78d43dbcef8119960ddc8090a73)

Git Hash: 7ee9798bad0b42983259e3a9e131789f4a49c117
Git Author: dim@FreeBSD.org

MFC r345703:
Fix initial exec TLS mode for dynamically loaded shared objects.

MFC r334505:
Don't export _end on arm64 and riscv.

MFC r314556-r314558

r314556:
Correct MDSRCS use in <arch>/string/Makefile.inc.

- Remove .c files which duplicate entries in MISRCS.
- Use the same, less merge conflict prone style in all cases.
- Use MDSRCS for mips (.c and .S files both ended up in SRCS).
- Remove pointless sparc64 Makefile.inc.
- Remove uninformative foreign VCS ID entries.

Reviewed by: emaste, imp, jhb
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D9841

r314557:
Correct an misunderstanding of MDSRCS.

MDSRCS it intended to allow assembly versions of funtions with C
implementations listed in MISRCS. The selection of the correct
machdep_ldis?.c for a given architecture does not follow this pattern
and the file should be added to SRCS directly.

Reviewed by: emaste, imp, jhb
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D9841

r314558:
Garbage collect unused gdtoa related files on mips.

Reviewed by: emase, imp, jhb
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D9841

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Replace use of the pipe(2) system call with pipe2(2) with a zero flags
value.

This eliminates the need for machine dependant assembly wrappers for
pipe(2).

It also make passing an invalid address to pipe(2) return EFAULT rather
than triggering a segfault. Document this behavior (which was already
true for pipe2(2), but undocumented).

Reviewed by: andrew
Approved by: re (gjb)
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D6815

Do not generate code for sbrk syscall -- sbrk support was removed.

Pointed out by: andrew

Remove legacy brk and sbrk from RISC-V.

Discussed with: andrew
Sponsored by: DARPA, AFRL
Sponsored by: HEIF5

libc: stop exporting curbrk and minbrk in the private namespace

They are not used anywhere else in the base system and are an internal
implementation detail that does not need to be exposed.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5728

Bring in initial libc and libstand support for RISC-V.

Reviewed by: andrew, emaste, kib
Sponsored by: DARPA, AFRL
Sponsored by: HEIF5
Differential Revision: https://reviews.freebsd.org/D4943

Start support for the RISC-V 64-bit architecture developed by UC Berkeley.

RISC-V is a new ISA designed to support computer research and education, and
is now become a standard open architecture for industry implementations.

This is a minimal set of changes required to run 'make kernel-toolchain'
using external (GNU) toolchain.

The FreeBSD/RISC-V project home: https://wiki.freebsd.org/riscv.

Reviewed by: andrew, bdrewery, emaste, imp
Sponsored by: DARPA, AFRL
Sponsored by: HEIF5
Differential Revision: https://reviews.freebsd.org/D4445