[MIPS] Simplify missing-syscalls for N32 and O32

Use standard missing-syscalls with EXTRA_CFLAGS instead of duplicating
the command.  And move the archprepare rule before the archclean rule.
Suggested by Franck Bui-Huu.  Also add "echo" to show the target ABI.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>