We can use xorps/xorpd on Intel CPUs to flip the sign bit. Ideally we'd
use an absolute 128-bit constant in the code object, as OCaml/GCC
does, however that requires 128-bit alignment for code objects,
which is not yet implemented. So for now we materialize the mask
inline.

As drive-by-fix, don't hardcode xmm0 as scratch double register.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/1064833002

Cr-Commit-Position: refs/heads/master@{#27618}