rustc: Don't error on the rlib symlinks

This commit implements a layman's version of realpath() for metadata::loader to
use in order to not error on symlinks pointing to the same file.

Closes #12459

src/test/run-make/symlinked-libraries/Makefile

@@ -0,0 +1,7 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) foo.rs
+	mkdir -p $(TMPDIR)/other
+	ln -nsf $(TMPDIR)/$(call DYLIB_GLOB,foo) $(TMPDIR)/other
+	$(RUSTC) bar.rs -L $(TMPDIR)/other

src/test/run-make/symlinked-libraries/bar.rs

@@ -0,0 +1,15 @@
+// Copyright 2012-2014 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.
+
+extern crate foo;
+
+fn main() {
+    foo::bar();
+}

src/test/run-make/symlinked-libraries/foo.rs

@@ -0,0 +1,13 @@
+// Copyright 2012-2014 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.
+
+#[crate_type = "dylib"];
+
+pub fn bar() {}