powerpc64: Fix by using the configure value $libc_cv_cc_submachine [BZ #31629]

This patch ensures that $libc_cv_cc_submachine, which is set from
"--with-cpu", overrides $CFLAGS for configure time tests.

Suggested-by: Peter Bergner <bergner@linux.ibm.com>
Reviewed-by: Peter Bergner <bergner@linux.ibm.com>