[mips][mips64r6] Add support for mips-img-linux-gnu GCC toolchains

Summary:
* Support the multilib layout used by the mips-img-linux-gnu
* Recognize mips{,64}{,el}-img-linux-gnu as being aliases of mips-img-linux-gnu
* Use the correct dynamic linker for mips-img-linux-gnu
* Make mips32r6/mips64r6 the default CPU for mips-img-linux-gnu

Subscribers: mpf

Differential Revision: http://reviews.llvm.org/D4436

llvm-svn: 212719