powerpc/64s/exception: denorm handler use standard scratch save macro

Although the 0x1500 interrupt only applies to bare metal, it is better
to just use the standard macro for scratch save.

Runtime code path remains unchanged (due to instruction patching).

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