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=

Review URL: https://codereview.chromium.org/1358553002

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