In 64 bit mode, allow 64 bit return values from clean helper calls.
This makes SMC checking calls work (even though they are irrelevant
on PPC targets).  Fixes #309430.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2793 8f6e269a-dfd6-0310-a8e1-e2731360e62c