Force to inline syscall_impl on all platforms (#186849)

With currently only LIBC_INLINE, we just hint the compiler to inline the
function which however in practice is not always the case.

Since we added `[[gnu::always_inline]]` on linux/x86_64 it makes sense
to do it on all platforms consistently and add a comment explaining why
we need it.

NOKEYCHECK=True
GitOrigin-RevId: 952dc121244765e6d2b5614b51d7293ab7a2f232