Shorten linker output even more when --verbose is not present

- Don't show environment variables. Seeing PATH is almost never useful, and it can be extremely long.
- For .rlibs in the sysroot, replace crate hashes with a `"-*"` string. This will expand to the full crate name when pasted into the shell.
- Move `.rlib` to outside the glob.
- Abbreviate the sysroot path to `<sysroot>` wherever it appears in the arguments.

This also adds an example of the linker output as a run-make test. Currently it only runs on x86_64-unknown-linux-gnu, because each platform has its own linker arguments. So that it's stable across machines, pass BUILD_ROOT as an argument through compiletest through to run-make tests.

- Only use linker-flavor=gnu-cc if we're actually going to compare the output. It doesn't exist on MacOS.

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -1007,6 +1007,7 @@ fn link_natively(
                     command: cmd,
                     escaped_output,
                     verbose: sess.opts.verbose,
+                    sysroot_dir: sess.sysroot.clone(),
                 };
                 sess.dcx().emit_err(err);
                 // If MSVC's `link.exe` was expected but the return code