Rewrite macOS SDK selection

Modern Xcode versions only have a single macOS SDK installed anymore, so
we don't need to iterate through our supported versions, only check
whether the current version is supported.

This also makes it so that we can specify "11" (or soon, "12") to
specify all 11.x SDKs, instead of having to update our supported list
multiple times a year. This matches previous years that had multiple
patch releases on 10.14.y, 10.15.y, etc.

Bug: 203607969
Test: build on Mac, changing the supported SDK list
Change-Id: I42715f7e8dfec811de440a435927c14c742dede9