Memoryperjob to limit number of proc and threads passed to qemu

[michelmno]

This feature request is from a suggestion of Stefan Brün expecting

• memoryperjob "adjusting the core count correctly" (as per sr#881507 & 881295 comments)
• "The memoryperjob constraint adjusts the number of cores available, by passing the appropriate number to the qemu invocation" (as per bug#1182757 comment 4)

As reminder, and as already summarized in bug#1182757 for build_limit alternative:

• build_limit macro from memory_constraint package
  https://build.opensuse.org/package/show/network:chromium/memory-constraints
• original memoryperjob implementation:
  for _constraints: memory requirements vary with available # or cores #1953
  [backend] support memory per job constraint #10008
• issue memoryperjob make no compliant worker for ppc64 not ppc64le
  why new "memoryperjob" make no compliant worker for ppc64 not ppc64le ? #10093
• use build_limit rather than memoryperjob
  https://bugzilla.opensuse.org/show_bug.cgi?id=1182757
• two sr that proposed to use "build_limit" macro that are on hold expecting this feature to be implemented:
  https://build.opensuse.org/request/show/881295 openscad declined for that
  https://build.opensuse.org/request/show/881507 blender

[StefanBruens]

This is not a feature request, but a request to make memoryperjob also work correctly on PPC.

This works correctly on all archs but PPC64, which is the only one which passes a separate thread/core count to qemu.