[libc] Avoid host header collisions in full builds (-nostdinc) (#187025)

When building the full library with -nostdinc, directly including
<stdint.h> may pull in host or compiler-provided headers that collide
with LLVM-libc's local macro definitions. Switch to using our internal
stdint-macros.h when LIBC_FULL_BUILD is enabled.

Additionally, declare aligned_alloc with noexcept in C++ to match common
C library declarations and avoid fatal type specification mismatches
during sysroot builds.

NOKEYCHECK=True
GitOrigin-RevId: bed77a1d9bf40c1ab0a55572df148e5a8c44e975