Allow externalizing strings in old pointer space.

This is what I think is a better solution to the "external strings in
old pointer space" problem. Basically, it is an issue because GC scans
all fields of objects in old pointer space and if the cached address
of the backing store is unaligned, it looks like a heap object, boom.

The solution here is to use short external strings when we externalize
a string in old pointer space, and when the address is unaligned.
Short external strings don't cache the address, so GC has no issues.

BUG=268686
LOG=Y
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/146183006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19093 ce2b1a6d-e550-0410-aec6-3dcde31c8c00