powerpc/kbuild: move -mprofile-kernel check to Kconfig

This eliminates the workaround that requires disabling
-mprofile-kernel by default in Kconfig.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>