[ELF] Split up emulation.s per backend

emulation.s is testing multiple architectures, which means it needs all
the corresponding backends enabled, which might not be true for all
developers (for example, I don't have PPC or MIPS enabled). Rather than
marking the entire test as unsupported for such developers, split it up
per backend to get better testing granularity.

Differential Revision: https://reviews.llvm.org/D53544

llvm-svn: 344986