AArch64: use MOVI instead of FMOV to zero FP register

HotSpot generates an FMOV from ZR to zero a floating point register:

   fmov d0, xzr

This used to be recommended by the Arm ARM, but that advice was removed in revision A.j and subsequent revisions (section C.5.3).

Integer->FP moves may be slow on some cores. Instead the preferred instruction is MOVI with immediate zero:

   movi d0, #0x0

Some micro-architectures special-case FMOV from ZR, and on those cores this change will have no effect, but in any case FMOV won't be faster than MOVI.