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>

package/libpfm4/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_LIBPFM4
 	bool "libpfm4"
 	depends on !BR2_avr32 # no perf_event_open
+	depends on !BR2_xtensa && !BR2_bfin # no nptl or linuxthreads (new)
 	depends on BR2_TOOLCHAIN_HAS_THREADS
 	help
 	  A helper library to help encode Performance Events to use with
@@ -10,3 +11,4 @@ config BR2_PACKAGE_LIBPFM4
 
 comment "libpfm4 needs a toolchain w/ threads"
 	depends on !BR2_TOOLCHAIN_HAS_THREADS
+	depends on !BR2_avr32 && !BR2_xtensa && !BR2_bfin