This CL introduces TurboAssembler, a super-class of Assembler and sub-class
of MacroAssembler. TurboAssembler contains all the functionality that is used
by Turbofan and previously was part of MacroAssembler. TurboAssembler has
access to the isolate but, in contrast to MacroAssembler, does not expect to
be running on the main thread.

Bug: v8:6048
Change-Id: If5693f56a45fe057e5011168e830d01a3f2f772d
Reviewed-on: https://chromium-review.googlesource.com/559674Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46477}