[tilegx] Implement MathPowStub.

  fixed a couple of new found Smi->Int conversion bug.
[tilegx] kEndOfChain should be zero for TileGX.

  1:
    b 1b

  above is a invalid branch. for mips hardware,
  the base pc to calculate target pc is pos_of_label_1 + 4,
  while for tilegx, the hardware don't have the +4 adjustment.

  So kEndOfChain should be zero, no need to do compensation.