stdlib: Improve fortify with clang

It improve fortify checks for realpath, ptsname_r, wctomb, mbstowcs,
and wcstombs.  The runtime and compile checks have similar coverage as
with GCC.

Checked on aarch64, armhf, x86_64, and i686.
Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>