powerpc/64: Rearrange 64-bit _switch to prepare for 32/64 merge

More some 64-bit specifics out from the function epilogue and rearrange
this to be a bit neater, use 32-bit mem ops for CR save/restore, and
change some register numbers.

This is preparation to consolidate 32-bit and 64-bit switch code.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230606132447.315714-3-npiggin@gmail.com