[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
Feeding isSafeToSpeculativelyExecute its DataLayout pointer

isSafeToSpeculativelyExecute can optionally take a DataLayout pointer. In the
past, this was mainly used to make better decisions regarding divisions known
not to trap, and so was not all that important for users concerned with "cheap"
instructions. However, now it also helps look through bitcasts for
dereferencable loads, and will also be important if/when we add a
dereferencable pointer attribute.

This is some initial work to feed a DataLayout pointer through to callers of
isSafeToSpeculativelyExecute, generally where one was already available.