[Powerpc darwin] AsmParser Base implementation.

This is a base implementation of the powerpc-apple-darwin asm parser dialect.

* Enables infrastructure (essentially isDarwin()) and fixes up the parsing of asm directives to separate out ELF and MachO/Darwin additions.
* Enables parsing of {r,f,v}XX as register identifiers.
* Enables parsing of lo16() hi16() and ha16() as modifiers.

The changes to the test case are from David Fang (fangism).