Rollup merge of #52006 - Mark-Simulacrum:keep-stage-fix, r=alexcrichton

 Change --keep-stage to apply more often

Previously, the --keep-stage argument would only function for compilers
that were depended on by future stages. For example, if trying to build
a stage 1 compiler you could --keep-stage 0 to avoid re-building the
stage 0 compiler. However, this is often not what users want in
practice.

The new implementation essentially skips builds all higher stages of the
compiler, so an argument of 1 to keep-stage will skip rebuilds of the
libraries, just linking them into the sysroot. This is unlikely to work
well in cases where metadata or similar changes have been made, but is
likely fine otherwise.

This change is somewhat untested, but since it shouldn't have any effect
except with --keep-stage, I don't see that as a large problem.

r? @alexcrichton
cc @nikomatsakis - I believe you wanted this functionality