powerpc/64s: Always define arch unmapped area calls

To avoid any functional changes to radix paths when building with hash
MMU support disabled (and CONFIG_PPC_MM_SLICES=n), always define the
arch get_unmapped_area calls on 64s platforms.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211201144153.2456614-16-npiggin@gmail.com