The actual number of double regs available on arm depends on the
current CPU. In some cases, we care about the max count of double regs
(e.g.  around Save/RestoreFPRegs which always reserves spaces for the
max); in others, we care about the actual count.

Use DwVfpRegister::kNumRegisters to fetch the max (= 32).
Use DwVfpRegister::SupportedRegisterCount() for the actual count (16
or 32).

Bug: v8:11062
Change-Id: I7d0c3f1263433ef5dc8efa69a68dff92a7168ae6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2501842Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70795}