support -mcpu=baseline, both in stage1 and stage2

See e381a42de9c0f0c5439a926b0ac99026a0373f49 for more details.
This is set up so that if we wish to make "baseline" depend on the
OS in the future, it is possible to do that.
0f016b368dAndrew Kelley committed on 2/20/2020, 11:31:17 PMparent33c69d5
6 files changedLine totals unavailable