unistd: Improve fortify with clang

It improve fortify checks for read, pread, pread64, readlink,
readlinkat, getcwd, getwd, confstr, getgroups, ttyname_r, getlogin_r,
gethostname, and getdomainname.  The compile and runtime 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>