kernel: use -ffunction-sections, -fdata-sections and --gc-sections on ARM to...

kernel: use -ffunction-sections, -fdata-sections and --gc-sections on ARM to reduce kernel image size, disable those flags when tracing on MIPS to fix compile errors

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35936 3c298f89-4303-0410-b956-a3cf2f4a3e73