libpfm4: disable when pthreads barrier support in missing

The old linuxthreads pthreads implementation does not support barriers.
Disable libpfm4 for arches that don't have either NPTL or new linuxthreads
pthreads implementation.

Fixes
http://autobuild.buildroot.net/results/ea5/ea594807b891cbe1f0eddad7f89d34af160636c4/

.

Also, add missing comment arches dependency.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>