arm64: change the representation of FPSR.QC so that it can be
used efficiently to record SIMD saturation, and remove support
for all other bits of FPSR, since we don't model them anyway.


git-svn-id: svn://svn.valgrind.org/vex@2894 8f6e269a-dfd6-0310-a8e1-e2731360e62c