Add to the VexAbiInfo structure, two new fields:

Add to the VexAbiInfo structure, two new fields:
  guest_amd64_assume_fs_is_zero
  guest_amd64_assume_gs_is_0x60

and use them to properly enable %fs/%gs prefix decoding for
guest-amd64.  This is needed to support amd64-darwin cleanly.

Unfortunately the VexAbiInfo needs to be plumbed to every single where
an address is decoded, which means the patch is vast, although very
trivial.

git-svn-id: svn://svn.valgrind.org/vex/trunk@1875