powerpc/signal_32: Simplify loop in PPC64 save_general_regs()

save_general_regs() which does special handling when i == PT_SOFTE.

Rewrite it to minimise the specific part, especially the __put_user()
and associated error handling is the same so make it common.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Use a regular if rather than ternary operator]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/47a38df46cae5a5a88a558a64d71f75e9c4d9950.1594125164.git.christophe.leroy@csgroup.eu