Make cpu_set_syscall_retval common between the existing FreeBSD ABI and
the Linuxulator. We need to translate error values onto Linux errno values
and return them to userspace when a syscall fails. We also need to preserve
x1 as all registers are preserved other than the return value.

Reviewed by: emaste
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16008