port 3e7e3ed7 (r32508)

  original commit message:
  * Add a sibling interface to InterpreterAssembler called
    CodeStubAssembler which provides a wrapper around the
    RawMachineAssembler and is intented to make it easy to build
    efficient cross-platform code stubs. Much of the implementation
    of CodeStubAssembler is shamelessly stolen from the
    InterpreterAssembler, and the idea is to eventually merge the
    two interfaces somehow, probably moving the
    InterpreterAssembler interface over to use the
    CodeStubAssembler. Short-term, however, the two interfaces
    shall remain decoupled to increase our velocity developing the
    two systems in parallel.
  * Implement the StringLength stub in TurboFan with the new
    CodeStubAssembler. Replace and remove the old Hydrogen-stub
    version.
  * Remove a whole slew of machinery to support JavaScript-style
    code stub generation, since it ultimately proved unwieldy,
    brittle and baroque. This cleanup includes removing the shared
    code stub context, several example stubs and a tangle of build
    file changes.

BUG=

Review URL: https://codereview.chromium.org/1492213002

Cr-Commit-Position: refs/heads/master@{#32538}