bsd/aarch64: clean-up atomics

CBNZ instruction doesn't affect condition flags, so there is no sense to
clobber them.

Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Vladimir Murzin <murzin.v@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
arch/aarch64: fix storage size for fpsr and fpcr

FPSR and FPCR registers are 32-bit wide.

Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Vladimir Murzin <murzin.v@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>