KGDB fixes for amd64 and aarch64.

[jhb]

Summary

• Update the amd64 kernel support to recognize mchk_calltrap as a trapframe generator.
• Remove some unneeded headers from fbsd-kld.c.
• Various fixes to get stack traces working for aarch64 kernels:
  • Map the LR register from the PCB to PC instead of LR.
  • Skip the PC register from the PCB as it isn't initialized to anything.
  • Correct the register cache map for the PCB. The old one had the offsets of pcb_sp and pcb_pc reversed.
  • Don't map all of the saved general purpose registers in a trapframe to X1 rather than X0 .. X29.
  • Use correct name for el0 trapframe entry points.

Test Plan

• Actually verified stack traces from kgdb on my rpi this time.

[pizzamig]

look good to me