linux + AArch64 + qemu + ffmpeg + NEON + debugging

- install crosstool-ng (http://crosstool-ng.org/, AUR package for arch users is working)
- go into a build directory, run `ct-ng menuconfig`
- create a profile with arch=arm, bitness=64bit, os=linux, tools=gdb
- ct-ng build
- add ~/x-tools/... to PATH, with for example:
    for d in $HOME/x-tools/*; do
      [ -d $d ] && export PATH=$d/bin:$PATH
    done
- ffmpeg/configure --cross_prefix=aarch64-unknown-linux-gnueabi-
                   --arch=aarch64
                   --target_os=linux
                   --target_exec="qemu-aarch64 -L $HOME/x-tools/aarch64-unknown-linux-gnueabi/aarch64-unknown-linux-gnueabi/sysroot/"
                   --samples=...
   note: target_exec is optional and useful only for FATE
- make ffmpeg ...
- qemu-aarch64 -g 2000 -L $HOME/x-tools/aarch64-unknown-linux-gnueabi/aarch64-unknown-linux-gnueabi/sysroot/ ./ffmpeg_g ...
- aarch64-unknown-linux-gnueabi-gdb ./ffmpeg_g -ex "target remote :2000" -ex "b ff_nv12_to_bgra_neon_16" -ex "c"