Better xz compression

Borrowed from downstream, we can be a little bit smarter
on how we compress modules.

kernel.spec

@@ -51,8 +51,6 @@ Summary: The Linux kernel
 
 %if %{zipmodules}
 %global zipsed -e 's/\.ko$/\.ko.xz/'
-# for parallel xz processes, replace with 1 to go back to single process
-%global zcpu `nproc --all`
 %endif
 
 # define buildid .local
@@ -2070,7 +2068,7 @@ find Documentation -type d | xargs chmod u+w
     fi \
   fi \
   if [ "%{zipmodules}" -eq "1" ]; then \
-    find $RPM_BUILD_ROOT/lib/modules/ -type f -name '*.ko' | xargs -P%{zcpu} xz; \
+    find $RPM_BUILD_ROOT/lib/modules/ -type f -name '*.ko' | %{SOURCE79} %{?_smp_mflags}; \
   fi \
 %{nil}