rules.mk: Create TARGET_ASFLAGS; force -mno-mips16

Create and use a TARGET_ASFLAGS, defaulting to TARGET_CFLAGS.

MIPS .S files reasonably assume they are not in mips16 mode. Because
"-mips16 -mno-mips16" results in -mno-mips16, I can append that to the
TARGET_ASFLAGS. This should be done with $(filter-out)?

Signed-off-by: Jay Carlson <nop@nop.com>
Signed-off-by: Florian Fainelli <florian@openwrt.org>

SVN-Revision: 36197