Rewrite arm/stack_machdep.c for EABI; add stack(9) support to arm kernels.

The old stack_machdep.c code was written for the APCS ABI (aka "oldabi").
When we switched to ARM EABI (back in freebsd 10) this file never got
updated, and apparently nobody noticed that until now.

The new implementation uses the same stack unwinder code used by the
arm implemenation of the db_trace stuff.