Clean up RegisterContextPOSIX i386 code.
Use 32-bit register enums without gaps on 64-bit hosts.
Don't show 64-bit registers when debugging 32-bit targets.
Add psuedo gpr registers (ax, ah, al, etc.)
Add mmx registers.
Fix TestRegisters.py to not read ymm15 register on 32-bit targets.
Fill out and move gcc/dwarf/gdb register enums to RegisterContext_x86.h