This refactors the logic from within the FastNewObject TF_BUILTIN to a
helper method which can be reused in other assemblers. This saves the
overhead of setting up the stub and calling into it.

A wrapper method is created for functions that don't need to tail call
into the runtime.

PromiseBuiltinsAssembler and RegexpBuiltinsAssembler are refactored to
use EmitFastNewObject.

Review-Url: https://codereview.chromium.org/2607233002
Cr-Commit-Position: refs/heads/master@{#42000}