powerpc/32: Remove __main()

Comment says that __main() is there to make GCC happy.

It's been there since the implementation of ppc arch in Linux 1.3.45.

ppc32 is the only architecture having that. Even ppc64 doesn't have it.

Seems like GCC is still happy without it.

Drop it for good.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d01028f8166b98584eec536b52f14c5e3f98ff6b.1623172922.git.christophe.leroy@csgroup.eu