Add test cases for VCVT.F64.{SU}32, #imm, including a bunch of dodging and
weaving to work around buggy assemblers.  Pertains to #308717.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13312 a5019735-40e9-0310-863c-91ae7b9d1cf9