X87: [builtins] Simplify String constructor code.

X87: [builtins] Simplify String constructor code.

port eadfd666318c1dd959d7fac7c27538ef478a8b72 (r30706).

original commit message:

    The String constructor was somewhat complex with a lot of micro
    optimizations that are not relevant or even misguided. It would be
    really hard to port that code to ES6, which requires String to be
    subclassable. So as a first step we reduced the necessary complexity
    to the bare minimum (also removing the last user of the fairly complex
    MacroAssembler::LookupNumberStringCache method).

    This also removes the counters for the String constructor, which
    were not properly exposed anymore (and not kept in sync with inlined
    versions of the String constructor anyway).

BUG=

Review URL: https://codereview.chromium.org/1336133003

Cr-Commit-Position: refs/heads/master@{#30744}