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>

openvdb.spec

@@ -112,6 +112,11 @@ sed -i \
 # https://bugzilla.redhat.com/show_bug.cgi?id=2021376
 %global optflags %(echo %{optflags} | sed 's/-g /-g1 /')
 %endif
+%if 0%{?rhel}
+%ifarch ppc64le s390x
+%undefine _smp_mflags
+%endif
+%endif
 export CXXFLAGS="%{build_cxxflags} -Wl,--as-needed"
 
 # Ignore versions (python 3, etc.)