ar71xx: fix build of lzma-loader with binutils 2.25.1

ar71xx: fix build of lzma-loader with binutils 2.25.1

To make the lzma-loader build with binutils 2.25.1 it is needed to
remove the .MIPS.abiflags section otherwise I get the following compile
error:

mips-openwrt-linux-musl-objcopy -O binary -R .reginfo -R .note -R .comment -R .mdebug -S loader loader.bin
mips-openwrt-linux-musl-objcopy: Warning: Writing section `.text' to huge (ie negative) file offset 0xffffffff80a00000.
mips-openwrt-linux-musl-objcopy:loader.bin[.text]: File truncated

This should close #20487.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46893 3c298f89-4303-0410-b956-a3cf2f4a3e73