PPC: [builtins] Simplify String constructor code.

PPC: [builtins] Simplify String constructor code.

Port eadfd666318c1dd959d7fac7c27538ef478a8b72

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).

R=bmeurer@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, dstence@us.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1339143002

Cr-Commit-Position: refs/heads/master@{#30722}