powerpc: Export msr_check_and_set() to modules

PR KVM will need to reuse msr_check_and_set().
This patch exports this API for reuse.

Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Reviewed-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>