Add some bits to support installing Ruby properly on Apple M1

[johnewart]

More fixes coming soon; this lets Ruby 2.7.x and earlier compile correctly on M1

[zombiezen]

Would you mind running the following on your M1 machine:

cd internal/buildpack && go test -run=TestRuby -record

The test failure is because the darwin/amd64 test wasn't re-recorded and it's detecting that the set of commands has changed. I can re-generate that file for you on my Mac mini.