For those using native clang/llvm for mips:
I'm able to start with llvm/clang on x86 linux and build a native clang/llvm/mips.
Then using this native clang/llvm/mips I can compile and link clang/llvm and the resulting compiler passes make check.
I'm doing more testing now with test-suite and other things.
No tweaking of scripts is necessary.
Just standard configure and make with the proper settings.