aarch64: implement lock adapter for arch_fpu

this fixes AArch64 build broken by #202b2ccc

Cc: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>