Implement C23 memset_explicit (bug 32378)

[Collin Funk]

Joseph Myers <josmyers@redhat.com> writes:

> +void *
> +__memset_explicit_chk (void *dst, int c, size_t len, size_t dstlen)
> +{
> +  /* Inline __memset_chk to avoid a PLT reference to __memset_chk.  */
> +  if (__glibc_unlikely (dstlen < len))
> +    __chk_fail ();
> +  memset (dst, c, len);
> +  /* Compiler barrier.  */
> +  asm volatile ("" ::: "memory");
> +  return dst;
> +}

Do we want the following workaround from Gnulib so Clang does not
optimize it away? Copying the snippet from there:

  memset (s, c, len);
  /* Compiler barrier.  */
  /* With asm ("" ::: "memory") LLVM analyzes uses of 's' and finds that the
     whole thing is dead and eliminates it.  Use 'g' to work around this
     problem.  See <https://bugs.llvm.org/show_bug.cgi?id=15495#c11>.  */
  __asm__ volatile ("" : : "g"(s) : "memory");
  return s;

Collin