Add initial support for MIPS.

At this point ./configure can complete successfully.
Make config/xclang respect --host and --build.

This allows us to use --host and --build to work around a bug in LLVM's
handling of mips* and mips64* triples. Currently, triples have an effect on
the output ELF that should be based on the ABI instead.
Make clang++ checks in configure.ac respect --host
Lower address spaces for Mips.

This fixes example1 (except SPIR variants), example2, and example2a among
others.
Merge branch 'mips' of github.com:dsandersimgtec/pocl