Fix: avoid -C prefer-dynamic for test harnesses of proc-macro crates

[tommantonclery]

Fixes #16000.

When running cargo test --no-run on a proc-macro crate with a dev-dependency on a dylib, Cargo was incorrectly passing -C prefer-dynamic to the test harness.

That caused rustc to try to link both rlib and dylib forms of std and its transitive crates, resulting in errors like:

error: cannot satisfy dependencies so `std` only shows up once

What changed

• Adjusted the logic in core/compiler so that -C prefer-dynamic is only applied when compiling an actual proc-macro crate.
• Test harnesses (even for proc-macro crates) no longer receive -C prefer-dynamic.

Why

This resolves the duplicate std/core/alloc linkage errors and makes cargo test --no-run consistent with cargo build.

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