[tilegx] Fix a critical bug in StringCompare.

  'left' operand is supposed to be kept in "a0", MIPS code use "v0"
as a scratch reg, while for TileGX, a0/v0 overlap, we need use "at"
as scratch to avoid reg clobbered unexpectedly.
[tilegx] Fix v0 clobber in WriteInt32ToHeapNumberStub.