Handle cycles with double stack slots in ARM64 parallel moves.

When acquiring a scratch register to emit a move between two
double stack slots, ask for a FP register first, to avoid
depleting the core scratch register pool, which is used in
vixl::aarch64::MacroAssembler::LoadStoreMacro when the
offset does not fit in the immediate field of the load
instruction.

Test: make test-art-target (on ARM64)
Bug: 34760542
Change-Id: Ie9b37d007ed6ec5886931a35dcb22a9aff73bbbe