powerpc/signal32: Move signal trampoline setup to handle_[rt_]signal32

Move signal trampoline setup into handle_signal32()
and handle_rt_signal32().

At the same time, remove the define which hides the mc_pad field
used for trampoline.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e439cc0fa35aa45da6776520777a61848b92fd4b.1597770847.git.christophe.leroy@csgroup.eu