Add write barrier helper for code patching and refactor stack check patching.

Add write barrier helper for code patching and refactor stack check patching.

The new helper avoids expensive FindCodeForInnerPointer invocation when we have
the host code object available. It is used when patching stack checks.

Also some comments on the ARM platform are corrected.
Committed: http://code.google.com/p/v8/source/detail?r=9687

[fschneider, 2011-10-18 14:59:24]

Small refactoring of stack-check patching code.

[Vyacheslav Egorov (Chromium), 2011-10-18 15:04:12]

LGTM!