This CL attempts to simplify the Assembler's dependency on the
isolate, in particular on a global "serializer_enabled" mode contained
therein. The "serializer_enabled" condition enabled and disabled
a number of things in both the assemblers and macro assemblers. To
make these dependencies explicit, the Assembler::IsolateData is refactored
to be a proper Assembler::Options struct that controls specific assembler
behaviors, with default settings easily computable from the isolate.

This also helps make the contract for compiling WASM code more explicit
(since WASM code needs to have reloc info recorded for external references)
we can explicitly enable this recording without trying to "trick" the
assembler using "serializer_enabled".

R=jgruber@chromium.org
CC=mstarzinger@chromium.org, herhut@chromium.org

Change-Id: I7a8ba49df7b75b292d73ec2aa6e507c27a3d99c8
Reviewed-on: https://chromium-review.googlesource.com/1105982
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53890}