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

Cherry-picked r241678 from upstream, modified it to the current
PNaCl-Clang tree, and added an extra flag "-direct-to-nacl" that is not
present upstream.

Original commit message:

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.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4205
R=dschuff@chromium.org

Review URL: https://codereview.chromium.org/1208073002 .

• lib/Basic/Targets.cpp[diff]
• lib/CodeGen/TargetInfo.cpp[diff]
• lib/Driver/ToolChains.cpp[diff]
• lib/Driver/ToolChains.h[diff]
• lib/Driver/Tools.cpp[diff]
• test/Driver/nacl-direct.c[diff]