powerpc/kernel: Use kprobe blacklist for asm functions

Rather than forcing the whole function into the ".kprobes.text" section,
just add the symbol's address to the kprobe blacklist.

This also lets us drop the three versions of the_KPROBE macro, in
exchange for just one version of _ASM_NOKPROBE_SYMBOL - which is a good
cleanup.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>