[libunwind] NFC: Use macros to accommodate differences in representation of PowerPC assemblers

Summary:
This NFC patch replaces the representation of registers and the left shift operator in the PowerPC assembly code to allow it to be consumed by the GNU flavored assembler and the AIX assembler.

* Registers - change the representation of PowperPC registers from %rn, %fn, %vsn, and %vrn to the register number alone, e.g., n. The GNU flavored assembler and the AIX assembler are able to determine the register kind based on the context of the instruction in which the register is used.

* Left shift operator - use macro PPC_LEFT_SHIFT to represent the left shift operator. The left shift operator in the AIX assembly language is < instead of <<

Reviewed by: sfertile, MaskRay, compnerd

Differential Revision: https://reviews.llvm.org/D101179