powerpc/32s: Reduce default size of module/execmem area

book3s/32 platforms have usually more memory than 8xx, but it is still
not worth reserving a full segment (256 Mbytes) for module text.
64Mbytes should be far enough.

Also fix TASK_SIZE when EXECMEM is not selected, and add a build
verification for overlap of module execmem space with user segments.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/c1f6a4e47f177d919561c6e97d31af5564923cf6.1724173828.git.christophe.leroy@csgroup.eu