Tweak and add test for detecting libraries with mismatching target triples.

Closes #10814.

src/test/run-make/mismatching-target-triples/Makefile

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+-include ../tools.mk
+
+# Issue #10814
+#
+# these are no_std to avoid having to have the standard library or any
+# linkers/assemblers for the relevant platform
+
+all:
+	$(RUSTC) foo.rs --target=i686-unknown-linux-gnu
+	$(RUSTC) bar.rs --target=x86_64-unknown-linux-gnu 2>&1 \
+		| grep "couldn't find crate .foo. with expected target triple x86_64-unknown-linux-gnu"

src/test/run-make/mismatching-target-triples/bar.rs

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+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+#![no_std]
+extern crate foo;

src/test/run-make/mismatching-target-triples/foo.rs

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+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+#![no_std]
+#![crate_type = "lib"]