Use the registers g1 and g5 as temporaries for making far jumps and far calls,

because saving i1 and i2 to their ``designated'' stack slots corrupts unknown
memory in other functions, standard libraries, and worse.

In addition, this has the benefit of improving JIT performance because we
eliminate writing out 4 instructions in CompilationCallback() and 2 loads and 2
stores.

llvm-svn: 7653