Add support for booting into kdb on arm64 platforms when RB_KDB is set (i.e. boot -d from a loader prompt).
EDIT: this is similar to a diff I submitted and was committed (r329989) for the arm platform. In that patch, I also fixed a problem in arm/arm/machdep_boot.c which was missing "#include opt_ddb.h" which meant none of the "#ifdef DDB" code gets compiled. I tried to fix arm64/arm64/machdep_boot.c in a similar fashion but it fails to compile due to MAGIC_TRAMP_NUMBER not being defined. I don't know the right way to fix that.