Make "Assemble stage1 compiler" orders of magnitude faster

This used to take upwards of 5 seconds for me locally. I found that the culprit was copying the downloaded LLVM shared object:
```
[22:28:03] Install "/home/jnelson/rust-lang/rust/build/x86_64-unknown-linux-gnu/ci-llvm/lib/libLLVM-14-rust-1.62.0-nightly.so" to "/home/jnelson/rust-lang/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libLLVM-14-rust-1.62.0-nightly.so"
[22:28:09]   c Sysroot { compiler: Compiler { stage: 1, host: x86_64-unknown-linux-gnu(x86_64-unknown-linux-gnu) } }
```

It turned out that `install()` used full copies unconditionally. Change it to try using a hard-link before falling back to copying.

Author: jyn514

src/bootstrap/lib.rs

@@ -1492,20 +1492,14 @@ impl Build {
         let dst = dstdir.join(src.file_name().unwrap());
         self.verbose_than(1, &format!("Install {:?} to {:?}", src, dst));
         t!(fs::create_dir_all(dstdir));
-        drop(fs::remove_file(&dst));
         {
             if !src.exists() {
                 panic!("Error: File \"{}\" not found!", src.display());
             }
-            let metadata = t!(src.symlink_metadata());
-            if let Err(e) = fs::copy(&src, &dst) {
-                panic!("failed to copy `{}` to `{}`: {}", src.display(), dst.display(), e)
-            }
-            t!(fs::set_permissions(&dst, metadata.permissions()));
-            let atime = FileTime::from_last_access_time(&metadata);
-            let mtime = FileTime::from_last_modification_time(&metadata);
-            t!(filetime::set_file_times(&dst, atime, mtime));
+            self.copy(src, &dst);
         }
+        // NOTE: when using hard-links, this will also update the permissions on the original file.
+        // We never use permissions that are more restrictive than the original, so this shouldn't cause any issues.
         chmod(&dst, perms);
     }