[libc] Silence warning about returning from noreturn function

[jhuber6]

Summary

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.

libc/src/stdlib/exit.cpp

 
 namespace internal {
 void call_exit_callbacks();
 }
 
 LLVM_LIBC_FUNCTION(void, exit, (int status)) {
   internal::call_exit_callbacks();
   quick_exit(status);
+  __builtin_unreachable();
 }
 
 } // namespace __llvm_libc