amd64: partially depessimize cpu_fetch_syscall_args and cpu_set_syscall_retval

Vast majority of syscalls take 6 or less arguments. Move handling of other
cases to a fallback function. Similarly, special casing for _syscall
and __syscall
magic syscalls is moved away.

Return is almost always 0. The change replaces 3 branches with 1 in the common
case. Also the 'frame' variable convinces clang not to reload it on each access.

Reviewed by: kib
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17542