Pass --target=$TARGET when running rustc in build.rs

I'm using this in an environment where only the `$TARGET` rust standard
library is available, which means `use std::arch::asm;` doesn't work for
the host platform when cross compiling, even when it works for the
target platform. Also in general, this is trying to detect things about
the target, not the host.

For context, I'm using [Bazel](https://bazel.build/) with
[rules_rust's cargo_build_script](http://bazelbuild.github.io/rules_rust/flatten.html#cargo_build_script),
via [cargo-raze](https://github.com/google/cargo-raze). Native
x86_64-unknown-linux-gnu compilation works fine without this patch, but
cross compiling to aarch64-unknown-linux-gnu requires this patch.

build.rs

@@ -174,10 +174,13 @@ fn can_compile(code: &str) -> bool {
     use std::process::Stdio;
     let out_dir = var("OUT_DIR").unwrap();
     let rustc = var("RUSTC").unwrap();
+    let target = var("TARGET").unwrap();
 
     let mut child = std::process::Command::new(rustc)
         .arg("--crate-type=rlib") // Don't require `main`.
         .arg("--emit=metadata") // Do as little as possible but still parse.
+        .arg("--target")
+        .arg(target)
         .arg("--out-dir")
         .arg(out_dir) // Put the output somewhere inconsequential.
         .arg("-") // Read from stdin.