powerpc/64: ELFv2 use minimal stack frames in int and switch frame sizes

Adjust the ELFv2 interrupt and switch frames to the minimum C ABI size,
plus pt_regs, plus 16 bytes for the aligned regs marker for the int
frame (and the switch frame needs to match that because it uses the same
regs offset as the int frame).

This saves 80 bytes of kernel stack per interrupt. It's the principle of
getting our accounting right that's more important than the practical
saving.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221127124942.1665522-17-npiggin@gmail.com