Disable smp builds for ppc64le and s390 epel
In koji, these builds are on low memory, multi core VM's
the compiling memory usage for a single file can be > 4G
which crashes the build when memory is exhausted.
So build with a single compile job by undef-ing _smp_mflags
Signed-off-by: Tom Rix <trix@redhat.com>