powerpc/irq: remove inline assembly in hard_irq_disable macro

Use WRITE_ONCE() instead of opencoding the saving of current
stack pointeur.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9f05937d8722ddd2064a7c2362d8f9000e15e1ba.1652863723.git.christophe.leroy@csgroup.eu