[Bug c/120850] provide an special __builtin_memset_explicit

[crrodriguez at opensuse dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120850

--- Comment #9 from Cristian Rodríguez <crrodriguez at opensuse dot org> ---
(In reply to Andrew Pinski from comment #2)
> Using an inline-asm to cause compiler barriers should be enough to stop gcc
> from optimizing the code away.

The library implementation does that. but callers wont know, the implementation
is not inline..