math: fix spurious overflow in pow with clang

clang does not support c99 fenv_access and may move fp operations out
of conditional blocks causing unconditional fenv side-effects. Here

  if (cond)
    ix = f (x * 0x1p52);

was transformed to

  ix_ = f (x * 0x1p52);
  ix = cond ? ix_ : ix;

where x can be a huge negative value so the mul overflows. The added
barrier should prevent such transformation by significantly increasing
the cost of doing the mul unconditionally.

Found by enh from google on android arm and aarch64 targets.
Fixes github issue #16.