arm64: rename guest_SP to guest_XSP so as to avoid a name clash with
guest_SP from s390 world.  Also back out the rename of guest_SP to
guest_s390_SP that caused s390 build breakage in r2803.



git-svn-id: svn://svn.valgrind.org/vex/trunk@2805 8f6e269a-dfd6-0310-a8e1-e2731360e62c