allow codegen targets (Architectures and Assemblers) to co-exist

The primary motivation behind this is to allow all the different Assemblers
to be built at once, on a single machine. This should dramatically reduce
the time required to make sure that a particular change doesn't break
the build for one of the not-so-common architectures (arm, powerpc)

Simply pass "codegen-targets=all" to make to compile all
src/codegen/<arch>/assembler.cpp.

Note that while these architectures are built, they will not be fully-
functional.  Certain stuff is assumed to be the same across the entire
build (such as TargetBytesPerWord), but this isn't the case anymore.
fix unused function warnings in codegen/powerpc/assembler.cpp when building with clang