powerpc/32s: Define a MODULE area below kernel text all the time

On book3s/32, the segment below kernel text is used for module
allocation when CONFIG_STRICT_KERNEL_RWX is defined.

In order to benefit from the powerpc specific module_alloc()
function which allocate modules with 32 Mbytes from
end of kernel text, use that segment below PAGE_OFFSET at all time.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a46dcdd39a9e80b012d86c294c4e5cd8d31665f3.1617283827.git.christophe.leroy@csgroup.eu