[libc] Introduce a full build mode CMake option.

This option will build LLVM libc as a full libc by itself. In this mode,
it is not expected that it will be mixed with other libcs. The
non-full-build mode will be the default LLVM libc build mode. In a future
where LLVM libc is complete enough, the full libc build will be made the
default mode.

GitOrigin-RevId: e9e788d145f57bcf815b9797191c1560ba39b842
8 files changed