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.
