powerpc/dtrace: Fix dtrace powerpc asm, and simplify stack walking

Fix some execution bugs in the dtrace powerpc asm. addme pulls in the carry
flag which we don't want, and the result wasn't recorded anyways, so the
following beq to check for exit condition wasn't checking the right
condition.

Simplify the stack walking in dtrace_isa.c, so there's only a single walker
that handles both pc and sp. This should make it easier to follow, and any
bugfix that may be needed for walking only needs to be made in one place
instead of two now.

MFC after: 2 weeks