x64: 'syscall' instruction support

Enable "fast system calls" via the 'syscall' instruction on OSv. The
instruction is used by Go programs on Linux/x86-64 for system calls.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
arch_prctl() system call

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>