Auto merge of #30948 - fabricedesre:rpi2, r=alexcrichton

This adds support for the armv7 crosstool-ng toolchain for the Raspberry Pi 2.

Getting the toolchain ready:
Checkout crosstool-ng from https://github.com/crosstool-ng/crosstool-ng
Build crosstool-ng
Configure the rpi2 target with |ct-ng armv7-rpi2-linux-gnueabihf|
Build the toolchain with |ct-build| and add the path to $toolchain_install_dir/bin to your $PATH

Then, on the rust side:
configure --target=armv7-rpi2-linux-gnueabihf && make && make install

To cross compile for the rpi2,
add $rust_install_path/lib to your $LD_LIBRARY_PATH, then use
rustc --target=armv7-rpi2-linux-gnueabihf -C linker=armv7-rpi2-linux-gnueabihf-g++ hello.rs