This allows WebAssembly runtime stubs implemented as {WasmCode} to be
called with regular stub linkage. So far we have only been able to call
such stubs with WebAssembly linkage.

Also switch two more on-heap builtins over to WebAssembly runtime stubs.

R=clemensh@chromium.org
BUG=v8:7424

Change-Id: Ifa553b5908ee27a1be780c325a114449d7fe7001
Reviewed-on: https://chromium-review.googlesource.com/1100882Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53734}