Imported Upstream version 5.0.0.42

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external/corert/Documentation/cross-building.md

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+Cross Compilation for ARM on Linux
+==================================
+
+Through cross compilation, on Linux it is possible to build CoreCLR for arm or arm64.
+
+Requirements
+------------
+
+You need a Debian based host and the following packages needs to be installed:
+
+    $ sudo apt-get install qemu qemu-user-static binfmt-support debootstrap
+
+In addition, to cross compile CoreCLR the binutils for the target are required. So for arm you need:
+
+    $ sudo apt-get install binutils-arm-linux-gnueabihf
+
+and conversely for arm64:
+
+    $ sudo apt-get install binutils-aarch64-linux-gnu
+
+
+Generating the rootfs
+---------------------
+The `cross\build-rootfs.sh` script can be used to download the files needed for cross compilation. It will generate an Ubuntu 14.04 rootfs as this is what CoreCLR targets.
+
+    Usage: build-rootfs.sh [BuildArch]
+    BuildArch can be: arm, arm64
+
+The `build-rootfs.sh` script must be run as root as it has to make some symlinks to the system, it will by default generate the rootfs in `cross\rootfs\<BuildArch>` however this can be changed by setting the `ROOTFS_DIR` environment variable.
+
+For example, to generate an arm rootfs:
+
+    $ sudo ./cross/build-rootfs.sh arm
+
+and if you wanted to generate the rootfs elsewhere:
+
+    $ sudo ROOTFS_DIR=~/coreclr-cross/arm ./build-rootfs.sh arm
+
+Cross compiling CoreCLR
+-----------------------
+Once the rootfs has been generated, it will be possible to cross compile CoreCLR. If `ROOTFS_DIR` was set when generating the rootfs, then it must also be set when running `build.sh`.
+
+So, without `ROOTFS_DIR`:
+
+    $ ./build.sh arm debug verbose clean cross
+
+And with:
+
+    $ ROOTFS_DIR=~/coreclr-cross/arm ./build.sh arm debug verbose clean cross
+
+As usual the resulting binaries will be found in `bin/Product/BuildOS.BuildArch.BuildType/`