Split out osMutator from archMutator

Split the archMutator into two mutators, osMutator to create
android/linux/darwin variants, and archMutator to then further
split them into arm/arm64, etc.  When combined with alias
variants in Blueprint this will allow us to depend on the
"android" variant without having to know what architectures
were used to build it.

Test: m checkbuild
Change-Id: I0f51d60fc440cf5ae503904ce7849ec797af5fe2