cpu_fetch_syscall_args becoms jmp-free for the common case (modulo sv_mask which is going away soon). Bumps getuid rate on Broadwell from 105mln to 107.5mln.
This pessimizes 6 or more arg syscalls, which are a complete minority.
cpu_set_syscall_retval change removes 3 branches in favor of 1 and prevents reloads of ->td_frame.