[MIPS] Add support for direct-to-nacl in Clang

For Mips direct-to-nacl, the goal is to be close to le32 front-end and
use Mips32EL backend. This patch defines new NaClMips32ELTargetInfo and
modifies it slightly to be close to le32. It also adds necessary parts,
inline with ARM and X86.

Differential Revision: http://reviews.llvm.org/D10739

llvm-svn: 241678