bootstrap: auto-patch libgccjit.so for NixOS

Currently all downloaded rustc and LLVM components are auto patched on
NixOS, but this is not done for libgccjit.so, so when GCC backend is
enabled on NixOS, the build ICEs with errors like this:

    thread 'rustc' (2286205) panicked at compiler/rustc_codegen_gcc/src/lib.rs:191:9:
    Cannot load libgccjit.so: libzstd.so.1: cannot open shared object file: No such file or directory

Fix this by auto-patch libgccjit.so, too. `zstd` is added to the dependency
environment.

Signed-off-by: Gary Guo <gary@garyguo.net>

Author: nbdd0121

src/bootstrap/src/core/download.rs

@@ -398,6 +398,16 @@ impl Config {
             self.download_file(&format!("{base}/{gcc_sha}/{filename}"), &tarball, help_on_error);
         }
         self.unpack(&tarball, root_dir, "gcc-dev");
+
+        if self.should_fix_bins_and_dylibs() {
+            let lib_dir = root_dir.join("lib");
+            for entry in t!(fs::read_dir(lib_dir)) {
+                let lib = t!(entry).path();
+                if path_is_dylib(&lib) {
+                    self.fix_bin_or_dylib(&lib);
+                }
+            }
+        }
     }
 }
 
@@ -677,6 +687,8 @@ fn fix_bin_or_dylib(out: &Path, fname: &Path, exec_ctx: &ExecutionContext) {
         // bintools: Needed for the path of `ld-linux.so` (via `nix-support/dynamic-linker`).
         // cc.lib: Needed similarly for `libstdc++.so.6`.
         // zlib: Needed as a system dependency of `libLLVM-*.so`.
+        // zstd.out: Needed as a system dependency of `libgccjit.so`. `.out` is necessary as the
+        //           default output of `zstd` derivation is `.bin`.
         // patchelf: Needed for patching ELF binaries (see doc comment above).
         let nix_deps_dir = out.join(".nix-deps");
         const NIX_EXPR: &str = "
@@ -685,6 +697,7 @@ fn fix_bin_or_dylib(out: &Path, fname: &Path, exec_ctx: &ExecutionContext) {
             name = \"rust-stage0-dependencies\";
             paths = [
                 zlib
+                zstd.out
                 patchelf
                 stdenv.cc.bintools
                 stdenv.cc.cc.lib