powerpc/64s: Move CPU -mtune options into Kconfig

Currently the -mtune options are set in the Makefile, depending on what
the compiler supports.

One downside of doing it that way is that the chosen -mtune option is
not recorded in the .config.

Another downside is that if there's ever a need to do more complicated
logic to calculate the correct option, that gets messy in the Makefile.

So move the determination of which -mtune option to use into Kconfig
logic.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230329234308.2215833-1-mpe@ellerman.id.au