Avoid scratch register exhaustion during ARM64 stack slot moves.

On ARM64, do not limit the selection of a scratch register
used in a move between two stack slots (or between two
double stack slots) to VIXL's floating-point register pool,
as it contains only one register (D31) and can be exhausted
in some rare cases. Instead, query both the core and the FP
register pools.

Add a regression run-test (626-checker-arm64-scratch-register).

Test: m test-art-target (on ARM64)
Bug: 32545705
Change-Id: I6203a4340e3c8b4f4879c07ed1be4c433c311c0f