kernel: Move the debug stuff into a common script

Move a copy of amd64's debug code into debug.ldscript. Make all the
kernels use this. This has the effect of modernizing the STABS for
powerpc as the others were almost already in sync. For the ones that
weren't this adds the DWARF 3 debug symbols from i386/amd64.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D44071