rust: Allow rustlibs to fallback to rlibs.

Allow adding rlib-only libraries to rustlibs by having it fallback to
selecting the rlib variant if a dylib variant is requested but not
available.

Bug: 224771894
Test: m nothing
Test: New Soong test
Change-Id: I47b19ec9dbf3da90a2b090aeda04556a0400c7d1