When running virtualised the powerpc kernel is able to run the system
in "compat mode" - which means the kernel and hardware are pretending
to userspace that the CPU is an older version than it actually is.

AT_BASE_PLATFORM is an AUXV entry that we export to userspace for use
when we're running in that mode, which tells userspace the "platform"
string for the real CPU version, as opposed to the faked version.

Although we don't support compat mode when using DT CPU features, and
arguably don't need to set AT_BASE_PLATFORM, the existing cputable
based code always sets it even when we're running bare metal. That
means the lack of AT_BASE_PLATFORM is a user-visible artifact of the
fact that the kernel is using DT CPU features, which we don't want.

So set it in the DT CPU features code also.

This results in eg:
  $ LD_SHOW_AUXV=1 /bin/true | grep "AT_.*PLATFORM"
  AT_PLATFORM:     power9
  AT_BASE_PLATFORM:power9
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>