moarvm: disable for sparc64

Missing implementation of AO_fetch_compare_and_swap in
libatomic_ops library for sparcv9.

Fixes:
http://autobuild.buildroot.net/results/268e328e2d9ea951c0728ab6031d32e2b460d015/
http://autobuild.buildroot.net/results/84847a9201244fce3e05e5e4e201f2e1848238de/



[Peter: add autobuilder references]
Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>