powerpc64: Fix stack setup in dbtrap

r330610 relocated the DMAP from the base of memory to the base of the fourth
quadrant of memory. This broke synthetic traps, such as KDB forced
breakpoints. Use GET_TOCBASE() so the DMAP offset is handled.

Submitted by: git_bdragon.rkt0.net
Differential Revision: https://reviews.freebsd.org/D15973