This is a first step towards moving Turbofan code generation off the main thread.

Summary of the changes:
- AssemblerBase no longer has a pointer to the isolate. Instead, its
  constructor receives the few things that it needs from the isolate (on most
  architectures this is just the serializer_enabled flag).
- RelocInfo no longer has a pointer to the isolate. Instead, the functions
  that need it take it as an argument.  (There are currently still a few that
  implicitly access the isolate through a HeapObject.)
- The MacroAssembler now explicitly holds a pointer to the isolate (before, it
  used to get it from the Assembler).
- The jit_cookie also moved from AssemblerBase to the MacroAssemblers, since
  it's not used at all in the Assemblers.
- A few architectures implemented parts of the Assembler with the help
  of a Codepatcher that is based on MacroAssembler.  Since the Assembler no
  longer has the isolate, but the MacroAssembler still needs it, this doesn't
  work anymore.  Instead, these Assemblers now use a new PatchingAssembler.

BUG=v8:6048

Review-Url: https://codereview.chromium.org/2732273003
Cr-Commit-Position: refs/heads/master@{#43890}