[public][ulib][dev] Add __FALLTHROUGH macro for warning suppression

GCC 7 enables -Wimplicit-fallthrough with -Wextra, so we'll get it.
Clang has it available, so start using it and clean up code accordingly.

The GCC feature will actually recognize some formulaic comments to
suppress the warning. But there's also an explicit syntax, and
C++17 added a standard one. So use that instead, via the new macro
__FALLTHROUGH in <zircon/compiler.h>.

TC-36 #comment Clean up code in preparation for more -Wimplicit-fallthrough

Change-Id: I28309c91c32db1313d52f4255836c5ea687e141f

[fvm] Add host & target side support for compressed sparse file

Change-Id: I2ba9350cfbc98e96de7fc0a7a253e9c560ba208b

[build] flatten the build

Previously we treated kernel/, system/, and third_party/ as
overlays on a shared namespace. This required the concept
of "canonical" module names, and a lot of complexity to ensure
that things didn't collide and the build worked.

This change gets rid of that, no longer passes -I to make,
so that include directives from our *.mk files do not magically
wildcard across various paths, etc.

The most user-visible change is that everywhere where a module
name is specified (MODULE_DEPS, MODULE_LIBS, etc), full module
names like kernel/lib/io or system/ulib/mxio must be used instead
of previously-allowed "short" names like lib/io and ulib/mxio.

The build output still has a similar shape, but the first segment
of the module path (kernel/, system/, or third_party/) is no
longer elided under $(BUILDDIR)

Change-Id: I525aba1da1c86eb7a86007bddc669f7eeebfedd5

[build][musl] refer to musl libc as ulib/c

This makes the build rules and the intermediate
output files easier to understand, without having
to modify the path where musl libc lives.

The biggest observeable difference is the build
intermediate files:

$ ls build-magenta-pc-x86-64/ulib/c
config-module.h libc.abi.h libc.abi.stamp libc.so libc.so.id libc.so.strip
libc.a libc.abi.o libc.a.opts libc.so.abi libc.so.opts third_party

which previously were at build-magenta-pc-x86/ulib/musl
and named libmusl.*

and the replacing of ulib/musl with ulib/c in the library
dependencies of all the rules.mk files (the vast bulk of
this change)

Change-Id: If9c5d522099d2c9ac5bdb666b8e67a2eb3d316ee

[lz4] Make LZ4 compile in magenta

* Fix include paths
* Skip compiling certain parts for kernel

Change-Id: I1d01a06d6310e1aa2cb82d32eeeed3dab3dfbcec

[ulib][lib] Add LZ4 compression library

* Source: https://github.com/lz4/lz4.git
* Revision: r131

Change-Id: I3b75f333c162e395f354305d48de9adf6bf72cae