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>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36197 3c298f89-4303-0410-b956-a3cf2f4a3e73