PPC: [stubs] Refactor StringCompareStub and use it for HStringCompareAndBranch.

PPC: [stubs] Refactor StringCompareStub and use it for HStringCompareAndBranch.

Port 8016547c8e6fde00fff0a1791f3c83b444d8af25

Original commit message:
    The StringCompareStub used to take its parameters on the (JavaScript)
    stack, which made it impossible to use in TurboFan. Actually
    StringCompareStub was currently completely unused. This changes the
    calling convention to something TurboFan compatible and introduces a
    CallInterfaceDescriptor for StringCompareStub. It also changes
    HStringCompareAndBranch to use the StringCompareStub instead of using
    the full blown CompareICStub for a stupid string comparison.

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/d4d2ea7b8a28d900aa74b9fa83fd0b5e6bb17b6c
Cr-Commit-Position: refs/heads/master@{#30837}

[MTBrandyberry, 2015-09-18 18:10:45]

The CQ bit was checked by mbrandy@us.ibm.com to run a CQ dry run

[commit-bot: I haz the power, 2015-09-18 18:10:57]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1358553002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1358553002/1

[commit-bot: I haz the power, 2015-09-18 18:32:57]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-18 18:32:58]

Dry run: This issue passed the CQ dry run.

[dstence, 2015-09-18 18:38:31]

lgtm

[MTBrandyberry, 2015-09-18 18:38:43]

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

[commit-bot: I haz the power, 2015-09-18 18:38:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1358553002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1358553002/1

[commit-bot: I haz the power, 2015-09-18 18:40:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-18 18:40:47]

Patchset 1 (id:??) landed as
https://crrev.com/d4d2ea7b8a28d900aa74b9fa83fd0b5e6bb17b6c
Cr-Commit-Position: refs/heads/master@{#30837}