We can use xorps/xorpd on Intel CPUs to flip the sign bit. Ideally we'd
use a RIP-relative 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.

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/1046893002

Cr-Commit-Position: refs/heads/master@{#27611}