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=

Committed: https://crrev.com/4ca74e4f80259fa4c669f6921171dd9e2b013748
Cr-Commit-Position: refs/heads/master@{#30722}

[michael_dawson, 2015-09-14 19:00:04]

On 2015/09/14 18:54:52, mtbrandyberry wrote:

lgtm

[MTBrandyberry, 2015-09-14 19:00:36]

The CQ bit was checked by mbrandy@us.ibm.com

[commit-bot: I haz the power, 2015-09-14 19:00:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1339143002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1339143002/1

[commit-bot: I haz the power, 2015-09-14 23:16:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-14 23:17:04]

Patchset 1 (id:??) landed as
https://crrev.com/4ca74e4f80259fa4c669f6921171dd9e2b013748
Cr-Commit-Position: refs/heads/master@{#30722}