ar71xx: set mips16 support, tune cflags for 34kc (works well on MIPS 24K as...

ar71xx: set mips16 support, tune cflags for 34kc (works well on MIPS 24K as well, but helps a lot for 34K and 74K)

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

SVN-Revision: 36600