Add a PPC inline asm constraint type for single CR bits

This adds support for the PPC "wc" inline asm constraint (used for allocating
individual CR bits). Support for this constraint type was recently added to the
LLVM PowerPC backend. Although gcc does not currently support allocating
individual CR bits, this identifier choice has been coordinated with the gcc
PowerPC team, and will be marked as reserved for this purpose in the gcc
constraints.md file.

Prior to this change, none of the multi-character PPC constraints were handled
correctly (the '^' escape character was not being added as required by the
parsing code in LLVM). This should now be fixed. I'll add tests for these other
constraints as support is added for them in the backend.