[tilegx] Fix fabs optimization glitch.

  for fabs/fabsf, previously we use

    bfins r, zero, 31/63, 31/63

  it's better to use:

    bfextu r, r, 0, 30/62

  because bfextu will zero the upper bits automatically.
[tilegx] Support -menable-no-nans.

  llvm support an option "-menable-no-nans", if the user is sure that
NaN will not be used in his program, then he could enable this option,
the backend could generate optimized code.

  for TileGX, when this option enabled, we do not check the unordered
bit.