Make truss work for CloudABI processes on aarch64.

This change copies over amd64-cloudabi64.c to aarch64-cloudabi.c and
adjusts it to fetch the proper registers on aarch64. To reduce the
amount of shared code, the errno conversion function is moved into a
separate source file.

Reviewed by: jhb, andrew
Differential Revision: https://reviews.freebsd.org/D4023