Add canadian cross building how to for RPi3 debug

doc/cross-build.md

@@ -0,0 +1,40 @@
+# Canadian-Cross-build rustc for arvm7 RPi3 to build for aarch64-unknown
+
+Japaric's explanations of [building rustc](https://www.reddit.com/r/rust/comments/5ag60z/how_do_i_bootstrap_rust_to_crosscompile_for_a_new/) with notes on used repos, namely `rust-buildbot` and crosstool-ng examples.
+
+Crosstool for gcc toolchain:  `git clone git@github.com:asymptotik/crosstool-arm-osx.git` -- this is shit, avoid!
+
+`https://stackoverflow.com/questions/50955843/linking-with-arm-linux-gnueabihf-gcc-failed-when-cross-compiling-a-rust-applic`
+
+Crosstool for llvm toolchain: https://medium.com/@zw3rk/making-a-raspbian-cross-compilation-sdk-830fe56d75ba
+
+```
+./configure --prefix="/Users/berkus/Hobby/Metta/cross-tools/rpi3-cross-llvm/prebuilt" \
+            --target=arm-linux-gnueabihf \
+            --enable-gold=yes \
+            --enable-ld=yes \
+            --enable-targets=arm-linux-gnueabihf \
+            --enable-multilib \
+            --enable-interwork \
+            --disable-werror \
+            --quiet
+make && make install
+```
+
+```
+mkdir sdkroot
+rsync -rzLR --safe-links \
+      pi@172.20.10.2:/usr/lib/arm-linux-gnueabihf \
+      pi@172.20.10.2:/usr/lib/gcc/arm-linux-gnueabihf \
+      pi@172.20.10.2:/usr/include \
+      pi@172.20.10.2:/lib/arm-linux-gnueabihf \
+      sysroot/
+```
+
+`cp /Users/berkus/Hobby/Metta/cross-tools/rpi3-cross-llvm/sysroot/usr/lib/gcc/arm-linux-gnueabihf/6.3.0/crt* /Users/berkus/Hobby/Metta/cross-tools/rpi3-cross-llvm/prebuilt/arm-linux-gnueabihf/lib/`
+
+`cp -r /Users/berkus/Hobby/Metta/cross-tools/rpi3-cross-llvm/sysroot/usr/lib/gcc /Users/berkus/Hobby/Metta/cross-tools/rpi3-cross-llvm/prebuilt/arm-linux-gnueabihf/lib/`
+
+`set -x PATH /Users/berkus/Hobby/Metta/cross-tools/rpi3-cross-llvm/prebuilt/bin $PATH`
+`./x.py build --host armv7-unknown-linux-gnueabihf --target aarch64-unknown-none --stage 1 src/libtest`
+