fix(build-std): always link to std when testing proc-macros (#14850)

### What does this PR try to resolve?

Fixes #14735

rust-lang/rust#131188 removes libstd.so from
sysroot so `cargo test -Zbuild-std` no longer links to it. That results
in a "Library not loaded: @rpath/libstd-[HASH].dylib" when testing a
proc macro.

This is a pretty niche use case, though it can be easily reproduced by
running `cargo test -Zbuild-std` in any proc-macro package. Like in
[serde-rs/serde](https://github.com/serde-rs/serde/tree/b9dbfcb4ac3b7a663d9efc6eb1387c62302a6fb4)
running it would fail.

This patch adds a special case that if it is `cargo run/test` against a
proc-macro, fill in std dynamic library search path for it.

### How should we test and review this PR?

```
CARGO_RUN_BUILD_STD_TESTS=1 cargo +nightly t --test build-std test_proc_macro
```

or

```
git clone https://github.com/serde-rs/serde
cd serde
git switch -d b9dbfcb4ac3b7a663d9efc6eb1387c62302a6fb4
cargo +nightly t --test build-std
```

### Additional information

Author: ehuss

src/cargo/core/compiler/compilation.rs

@@ -315,7 +315,10 @@ impl<'gctx> Compilation<'gctx> {
             // libs from the sysroot that ships with rustc. This may not be
             // required (at least I cannot craft a situation where it
             // matters), but is here to be safe.
-            if self.gctx.cli_unstable().build_std.is_none() {
+            if self.gctx.cli_unstable().build_std.is_none() ||
+                // Proc macros dynamically link to std, so set it anyway.
+                pkg.proc_macro()
+            {
                 search_path.push(self.sysroot_target_libdir[&kind].clone());
             }
         }

tests/build-std/main.rs

@@ -360,3 +360,33 @@ fn remap_path_scope() {
         )
         .run();
 }
+
+#[cargo_test(build_std_real)]
+fn test_proc_macro() {
+    // See rust-lang/cargo#14735
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                edition = "2021"
+
+                [lib]
+                proc-macro = true
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("test --lib")
+        .env_remove(cargo_util::paths::dylib_path_envvar())
+        .build_std()
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.0 ([ROOT]/foo)
+[FINISHED] `test` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+[RUNNING] unittests src/lib.rs (target/debug/deps/foo-[HASH])
+
+"#]])
+        .run();
+}