[Mono-list] JIT patch for stabs debugging

[Serge]

> but x86_mov_reg_imm() already emits a XOR if the argument is zer

BTW, I also noticed this optimization some time ago, and maybe it would be
better to remove it?
This may lead to subtle bugs because the XOR has effect on flags.
For example if emitting ADD/ADC sequence for 64-bit values.
Sure such bugs are easy to spot, but maybe it's better to make it explicit.
XOR can be encoded with x86_alu_reg_reg macro anyway, and check for zero
argument can be coded outside of the macro, if needed.

Sergey