[libc] Silence warning about returning from noreturn function

The `exit` entrypoint calls into `quick_exit` which is marked noreturn
in some cases. This will cause errors because we then have control flow
externally. This warning can be silenced by using a
`__builtin_unreachable` instruction accordingly.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D152323