This CL adds a first implementation of Liftoff, the new wasm baseline
compiler, for x64 and ia32. It currently supports the most important
i32 instructions and control instructions. Whenever it encounters an
instruction it does not support yet, it aborts.
In a subsequent CL, Liftoff will be called from the
WasmCompilationUnit, falling back to Turbofan compilation if the
baseline compiler bails out.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ifa78fb9d546dce72c241ff01a251dfa13cb31c1d
Reviewed-on: https://chromium-review.googlesource.com/716480
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48832}