Make 32-bit system calls end up in svc_handler().

The nice thing about ARM64 is that it's pretty elegant to install
separate trap/exception handlers for 32-bit and 64-bit processes. That
said, for all other architectures (e.g., i386 on amd64) we always let
32-bit counterparts go through the regular system call codepath. Let's
do the same on ARM64.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D13146