This adds support for f32.convert_{s,u}/i{32,64}.
On 32-bit platforms, i64 conversions are implemented by a call to a c
function. Since the signature of this c function is very different to
the currently used functions in Liftoff (in particular they contain an
out parameter), this CL requires a major refactoring of how Liftoff
generates c-calls.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Iffb4d0263ca1ca3eb9c6216344220322cda16062
Reviewed-on: https://chromium-review.googlesource.com/952122
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51841}