Rollup merge of #113710 - loongarch-rs:fix-rpath, r=clubby789

Fix rpath for libdir is specified

## What does this PR try to resolve?

When building the Rust toolchain with `--libdir=lib64`, the executable tools such as `rustc` cannot find shared libraries.

```bash
./configure --prefix=/ --libdir=lib64
DESTDIR=/tmp/rust ./x.py install
```

```
$ /tmp/rust/bin/rustc
rustc: error while loading shared libraries: librustc_driver-13f1fd1bc7f7000d.so: cannot open shared object file: No such file or directory
```

This issue is caused by the link args `-Wl,rpath` being different from `--libdir`.

```
$ readelf -d /tmp/rust/bin/rustc | grep RUNPATH
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN/../lib]
```

## How to resolve?

When setting the rpath, get it from sysroot libdir relative path.

After this patch:

```
$ readelf -d /tmp/rust/bin/rustc | grep RUNPATH
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN/../lib64]
```

Author: matthiaskrgr

src/bootstrap/builder.rs

@@ -1629,17 +1629,18 @@ impl<'a> Builder<'a> {
         // fun to pass a flag to a tool to pass a flag to pass a flag to a tool
         // to change a flag in a binary?
         if self.config.rpath_enabled(target) && util::use_host_linker(target) {
+            let libdir = self.sysroot_libdir_relative(compiler).to_str().unwrap();
             let rpath = if target.contains("apple") {
                 // Note that we need to take one extra step on macOS 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.
                 rustflags.arg("-Zosx-rpath-install-name");
-                Some("-Wl,-rpath,@loader_path/../lib")
+                Some(format!("-Wl,-rpath,@loader_path/../{}", libdir))
             } else if !target.contains("windows") && !target.contains("aix") {
                 rustflags.arg("-Clink-args=-Wl,-z,origin");
-                Some("-Wl,-rpath,$ORIGIN/../lib")
+                Some(format!("-Wl,-rpath,$ORIGIN/../{}", libdir))
             } else {
                 None
             };