kernel: fix module export stripping

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35319 3c298f89-4303-0410-b956-a3cf2f4a3e73
kernel: use -ffunction-sections, -fdata-sections and --gc-sections on mips

in combination with kernel symbol export stripping this significantly reduces
the kernel image size

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35320 3c298f89-4303-0410-b956-a3cf2f4a3e73