sstate.bbclass: Ensure machine specific stamps are only wiped for the current task

sstate.bbclass: Ensure machine specific stamps are only wiped for the current task

sstate was being a little too ethusiastic about removing stamp files and
was removing stamp files for other machines when it shouldn't have been.

This patch teaches sstate about machine specific stamp extensions and
allows it to only remove the current task's stampfiles.

Based on a patch from Phil Blundell <philb@gnu.org> with some tweaks
from me.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>