Add support for the new mips-mti-linux toolchain.

Summary:
This new toolchain uses primarily LLVM-based tools, eg. compiler-rt, lld,
libcxx, etc. Because of this, it doesn't require neither an existing GCC
installation nor a GNU environment. Ideally, in a follow-up patch we
would like to add a new --{llvm|clang}-toolchain option (similar to
--gcc-toolchain) in order to allow the use of this toolchain with
independent Clang builds. For the time being, we use the --sysroot
option just to test the correctness of the paths generated by the
driver.

Reviewers: atanasyan, dsanders, rsmith

Subscribers: jfb, tberghammer, danalbert, srhines, dschuff, cfe-commits

Differential Revision: http://reviews.llvm.org/D13340

llvm-svn: 249137