build: fix aarch64 default cpu selection

Not sure since when the issue emerged, but according to the current doc of gcc
and as, armv8-a is intended as argument of -march

The change will affect at the moment arm64 and layerscape/64b

Below is the relevant error messages when building toolchain

    Assembler messages:
    Error: unknown cpu `armv8-a'
    Error: unrecognized option -mcpu=armv8-a
    /home/yousong/git-repo/lede-project/lede/build_dir/toolchain-aarch64_armv8-a_gcc-5.4.0_musl/gcc-5.4.0/libgcc/libgcc2.c:1:0: error: unknown value 'armv8-a' for -mcpu

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>