rustbuild: Fix LC_ID_DYLIB directives on OSX

Currently libraries installed by rustbuild on OSX have an incorrect
`LC_ID_DYLIB` directive located in the dynamic libraries that are
installed. The directive we expect looks like:

    @rpath/libstd.dylib

Which means that if you want to find that dynamic library you should
look at the dylib's other `@rpath` directives. Typically our `@rpath`
directives look like `@loader_path/../lib` for the compiler as that's
where the installed libraries will be located. Currently, though,
rustbuild produces dylibs with the directive that looks like:

    /Users/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-mac/build/build/x86_64-apple-darwin/stage1-std/x86_64-apple-darwin/release/deps/libstd-713ad88203512705.dylib

In other words, the build directory is encoded erroneously. The compiler
already [knows how] to change this directive, but it only passes that
argument when `-C rpath` is also passed. The rustbuild system, however,
explicitly [does not pass] this option explicitly and instead bakes its
own. This logic then also erroneously didn't pass `-Wl,-install_name`
like the compiler.

[knows how]: 4a008cccaa/src/librustc_trans/back/linker.rs (L210-L214)
[does not pass]: 4a008cccaa/src/bootstrap/bin/rustc.rs (L133-L158)

To fix this regression this patch introduces a new `-Z` flag, `-Z
osx-rpath-install-name` which basically just forces the compiler to take
the previous `-install_name` branch when creating a dynamic library.
Hopefully we can sort out a better rpath story in the future, but for
now this "hack" should suffice in getting our nightly builds back to the
same state as before.

Closes #38430

src/bootstrap/bin/rustc.rs

@@ -158,6 +158,15 @@ fn main() {
         // to change a flag in a binary?
         if env::var("RUSTC_RPATH") == Ok("true".to_string()) {
             let rpath = if target.contains("apple") {
+
+                // Note that we need to take one extra step on OSX to also pass
+                // `-Wl,-instal_name,@rpath/...` to get things to work right. To
+                // do that we pass a weird flag to the compiler to get it to do
+                // so. Note that this is definitely a hack, and we should likely
+                // flesh out rpath support more fully in the future.
+                if stage != "0" {
+                    cmd.arg("-Z").arg("osx-rpath-install-name");
+                }
                 Some("-Wl,-rpath,@loader_path/../lib")
             } else if !target.contains("windows") {
                 Some("-Wl,-rpath,$ORIGIN/../lib")

src/librustc/session/config.rs

@@ -928,6 +928,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
           "print some statistics about MIR"),
     always_encode_mir: bool = (false, parse_bool, [TRACKED],
           "encode MIR of all functions into the crate metadata"),
+    osx_rpath_install_name: bool = (false, parse_bool, [TRACKED],
+          "pass `-install_name @rpath/...` to the OSX linker"),
 }
 
 pub fn default_lib_output() -> CrateType {

src/librustc_trans/back/linker.rs

@@ -207,7 +207,12 @@ impl<'a> Linker for GnuLinker<'a> {
         if self.sess.target.target.options.is_like_osx {
             self.cmd.args(&["-dynamiclib", "-Wl,-dylib"]);
 
-            if self.sess.opts.cg.rpath {
+            // Note that the `osx_rpath_install_name` option here is a hack
+            // purely to support rustbuild right now, we should get a more
+            // principled solution at some point to force the compiler to pass
+            // the right `-Wl,-install_name` with an `@rpath` in it.
+            if self.sess.opts.cg.rpath ||
+               self.sess.opts.debugging_opts.osx_rpath_install_name {
                 let mut v = OsString::from("-Wl,-install_name,@rpath/");
                 v.push(out_filename.file_name().unwrap());
                 self.cmd.arg(&v);