openpgm: disable on AVR32

openpgm doesn't build correctly on AVR32 using
gcc-4.2.2-avr32-2.1.5 toolchain: it is configured to call
intrinsic atomic functions not provided by the toolchain,
so they are propagated as unresolved external symbols in the
built openpgm libraries. This breaks programs that try to link
openpgm, because they do not know where to get those either. For
instance, it breaks building zeromq tests when PGM support is
selected.

This commit disables openpgm on AVR32 due to apparent absence of
interest in this package on that architecture and it breaking too
many test builds.

Fixes http://autobuild.buildroot.net/results/5a3261109ea63ba17375003eabd8b5d88757865f/


(at least)

Signed-off-by: Alexander Lukichev <alexander.lukichev@gmail.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>