rpm: limit build jobs by system memory on SUSE

rpm: limit build jobs by system memory on SUSE

43b441f9a3bc907c17d52385251001ffcd5d3ff9 removed a bunch of code which the SUSE
builds were relying on to avoid OOM. This commit brings back that code in
a much-streamlined form: the SUSE-specific %limit_build macro.

This also has the advantage of not breaking the build on older RPMs which only
know about %_smp_mflags, and not the newer %_smp_build_ncpus etc. macros.

Fixes: 43b441f9a3bc907c17d52385251001ffcd5d3ff9
Fixes: https://tracker.ceph.com/issues/49556
Signed-off-by: Nathan Cutler <ncutler@suse.com>