build: decouple the mips16 support flag from the toolchain

Add the flags from package.mk instead, and leave libc and gcc
unaffected.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37770 3c298f89-4303-0410-b956-a3cf2f4a3e73