s390: It is not necessary to save/restore the link register when
making a helper call. The link register needs to be saved when
switching between valgrind and client code and the dispatcher code
already does that. Julian suggested this change when he merged the
COMEM branch.
This saves between 6% and 13% of insns on the perf bucket. 
Runtime difference is within noise margin.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2676 8f6e269a-dfd6-0310-a8e1-e2731360e62c