Fix undefined behavior in WTF::equal() in StringImpl.h for i386/x86_64
<http://webkit.org/b/130283>
<rdar://problem/16281477>

Reviewed by Geoff Garen.

* wtf/text/StringImpl.h:
(WTF::loadUnaligned): Add template method.
(WTF::equal): Switch to using loadUnaligned<>().

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@165681 268f45cc-cd09-0410-ab3c-d52691b4dbfc