string: Improve fortify with clang

It improve fortify checks for strcpy, stpcpy, strncpy, stpncpy, strcat,
strncat, strlcpy, and strlcat.  The runtime and compile checks have
similar coverage as with GCC.

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