bitbake.conf: Ensure XZ_THREADS doesn't change sstate checksums

rpm output packages currently depend on the value of XZ_THREADS which
is ok if left to the default value but problematic if system limits
are set such as on the autobuilder.

Force the value to a specific one in the hashes for better sstate reuse
and consistent rpm task checksums.

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