x86 JIT: Sacrifice a register for a pointer to the MIPS context. Shrinks emitted x86 code considerably.

Nice in 64-bit, but might be a bit too much in 32-bit though... Needs testing.
Avoid an ifdef using PTRBITS
Merge pull request #6996 from hrydgard/x86-ctxreg

x86 JIT: Sacrifice a register for a pointer to the MIPS context. Shrinks emitted x86 code considerably.