message: Improve `libtock_runtime`'s error handling if it can't find an assembly toolchain.
  Previously, if `libtock_runtime`'s build script was unable to find a toolchain to build its external assembly with, its build would succeed then the process binary would fail to link. This change makes `libtock_runtime`'s build script panic with a useful error message instead.
diff: //! point) and linking it into the process binary. Requires out_dir to be added
  //! to rustc's link search path.
  +/// Compiles the external assembly and tells cargo/rustc to link the resulting
  +/// library into the crate. Panics if it is unable to find a working assembly
  +/// toolchain or if the assembly fails to compile.
  pub(crate) fn build_and_link(out_dir: &str) {
  use std::env::var;
  let arch = var("CARGO_CFG_TARGET_ARCH").expect("Unable to read CARGO_CFG_TARGET_ARCH");
  @@ -49,6 +52,8 @@ pub(crate) fn build_and_link(out_dir: &str) {
  return;
  }
  }
  +
  + panic!("Unable to find a toolchain for architecture {}", arch);
  }
  #[derive(Clone, Copy)]
  