pattern mode can clobber private library state and have false negatives on segment refs

Xref https://github.com/DynamoRIO/dynamorio/issues/1823

There is a general issue in DR on Linux ever since we put in segment
mangling where a tool can't just blindly copy a far memref opnd.

Pattern mode today happens to avoid this in common cases by not bothering
to instrument for opnd_is_abs_addr(). However, for a far non-absolute
reference, pattern blindly tries to de-ref gs: and is thus buggy: on a
write it will clobber priv lib state in addition to not doing its job of
checking the proper app mem ref.