Avoid using non-zero argument for __builtin_frame_address().

Building kernel with devel/powerpc64-gcc (6.2.0) yields the following error:
/usr/src/sys/powerpc/powerpc/db_trace.c:299:20: error: calling
'__builtin_frame_address' with a nonzero argument is unsafe
[-Werror=frame-address]

Work around this by dereferencing the frame address manually instead.

PR: 215600
Reported by: Mark Millard <markmi AT dsl-only DOT net>
MFC after: 2 weeks