syslog: Improve fortify with clang

It improve fortify checks for syslog and vsyslog.  The compile
and runtime hecks have similar coverage as with GCC.

The syslog fortify wrapper calls the va_arg version, since clang does
not support __va_arg_pack.

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