arm64: optimize set_syscall_retval()

Microoptimize set_syscall_retval() for arm64 by predicting
the return value to be zero. This is similar to what has
been done for other architectures

Reviewed By: emaste, mhorne
Differential Revision: https://reviews.freebsd.org/D26991

(cherry picked from commit fb8c2f743ab695f6004650b58bf96972e2535b20)