Better error messages for "build.sh" without an ARM toolchain.

When running "build.sh" targeting ARM64, but no toolchain is installed,
give a more useful error message about how to install it.

Change-Id: I94b3fe3390fb6c27bcafafdf0662f58afd0bd4e8

build.sh

diff --git a/build.sh b/build.sh
index d4313f8..c55707c 100755
--- a/build.sh
+++ b/build.sh
@@ -32,6 +32,17 @@
   usage;;
 esac
 
+# If building ARM, ensure we have the correct toolchain installed.
+if [ $ARCH = "arm64" ]; then
+  type aarch64-linux-gnu-gcc >/dev/null 2>/dev/null || {
+    echo "Required package gcc-aarch64-linux-gnu is not installed. To install"
+    echo "on Debian-based systems, run:"
+    echo
+    echo "  sudo apt install gcc-aarch64-linux-gnu"
+    exit 1
+  }
+fi
+
 declare -r OUT_DIR="out/${ARCH}"
 declare -r IMAGE_FILE="out/linux-tests-${ARCH}.img"