Create os and arch variants for bootstrap Go modules

AddFarVariationDependencies was broken, which allowed modules to add
dependencies on bootstrap Go modules using os and arch variations
even though boostrap Go modules did not have any variations.  Mutate
bootstrap Go modules in the os and arch mutators so that they have the
expected os and arch variations.

This relands Ida7bc75a51ab1d2d38a6be11f574399097380cc9 with fixes
to also mutate *bootstrap.goPackage modules so that they can find a
matching *bootstrap.goBinary module when creating their pluginFor
dependency, and to create a variation for linux_bionic when it is
enabled so that linux_bionic builds can package Go binaries.

Also requries https://github.com/google/blueprint/pull/320 to
support multiple variants of bootstrap Go modules.

Test: all soong tests
Change-Id: Ibc627aa262c298a076d171a4c21b0b9c2a7b0ada