powerpc: merge 32-bit and 64-bit _switch implementation

The _switch stack frame setup are substantially the same, so are the
comments. The difference in how the stack and current are switched,
and other hardware and software housekeeping is done is moved into
macros.

Generated code should be unchanged.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Tweak include orer to fix compile errors on some configs]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230606132447.315714-6-npiggin@gmail.com