This adds support for i32.reinterpret/f32, i64.reinterpret/f64,
f32.reinterpret/i32, and f64.reinterpret/i64.
On x64, all operations are straight-forward. On ia32, conversions from
or to i64 are done via the stack.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: If5562caf7367726904c6e405ad4fc5436d21144e
Reviewed-on: https://chromium-review.googlesource.com/962224Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51920}