Fix asm constraint checking for Nyuzi

[jbush001]

https://github.com/jbush001/NyuziToolchain/blob/d311f0e8ba10b492bcc8f78a16b3911847c238b8/tools/clang/lib/Basic/Targets.cpp#L8645

It looks like I copy/pasted constraints from another target, but most of these are not supported by backend (I suspect they are used by some generic tests), and the comments are wrong. Either remove these, or add support and tests for them (in both LLVM and Clang).