Implement dt(4) utrace support on amd64 and i386.

Implement dt(4) utrace support on amd64 and i386.

This adds stacktrace_save_utrace() to extract and save the userland stack
which is stubbed out on most archs. alpha and riscv64 do not even implement
dt(4) and stacktrace_save_at() so the stubs are excluded there.

Additionally add a new ioctl DTIOCGETAUXBASE which allows btrace to
fetch the AUX_BASE vallue from the AUX vector of a process.

OK mpi@ (some time ago) discussed with kettenis@