DRD: Handle Imbe_CancelReservation properly (clrex on ARM)

This was reported by Florian and was detected by analyzing the
compiler output with -Wswitch-enum enabled.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14549 a5019735-40e9-0310-863c-91ae7b9d1cf9