Fix get/set_fpcr_aarch64 (#44256)
On Aarch64, the `fpcr` register is 64bit wide, although the top 32bit
are currently unused and reserved for future usage. Nevertheless, we
should safe and restore the full 64 bit, not just 32 bit. This also
silences a compiler warning about this. Reference:
<https://developer.arm.com/documentation/ddi0595/2021-06/AArch64-Registers/FPCR--Floating-point-Control-Register>
