meson: update unsupported host/CPU messages

Unsupported CPU and OSes are not really going away, but the
project simply does not guarantee that they work.  Rephrase
the messages accordingly.  While at it, move the warning for
TCI performance at the end where it is more visible.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>