X64: Optimize access to external references.

X64: Optimize access to external references.

Access to an ExternalReference in non-serializable code will try to use
an offset relative to the root-array register.
Since the root-array is in the Heap object, and the Heap object is in
the Isolate object, there's a good chance that any external data field
is within a 32-bit offset of the root array register.
It falls back on the original behavior if the serializer is enabled,
if the root register isn't initialized or if the offset is not representable
as a 32-bit value.

Review URL: http://codereview.chromium.org/6716018

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7315 ce2b1a6d-e550-0410-aec6-3dcde31c8c00