Change the syntax for arch selecting and add os selecting

This changes the syntax from select(variant("arch"), {...}) to
select(arch(), {...}) to make it shorter and to make it clear that
we can restrict what variants you can select on.

Bug: 323382414
Test: m nothing --no-skip-soong-tests
Change-Id: Iafe05b5f455895313a563ed6292f4016b58180b0