[MIPS] Fix switching between 32/64-bit variants of r6 target triples

If clang driver gets 64-bit r6 target triple like `mipsisa64r6` and
additional option forces switching to generation of 32-bit code, it
loses r6 abi and generates 32-bit r2-r5 abi code.

```
$ clang -target mipsisa64r6-linux-gnu -mabi=32
```

This patch fixes the problem.

- Add optional `SubArchType` argument to the `Triple::setArch()` method.
- Implement generation of mips r6 target triples in the
  `Triple::getArchName()` method.

Differential Revision: https://reviews.llvm.org/D110514.diff