Fix run-make tests on musl hosts

On musl hosts, we already set -Ctarget-feature=-crt-static and
IS_MUSL_HOST=1 in compiletest. However, in order for the run-make tests
to compile fine on musl hosts, we need to propagate this flag in our
rustc invocations to ensure we can generate cdylibs.

Signed-off-by: Jens Reidel <adrian@travitia.xyz>

Author: Gelbpunkt

src/tools/run-make-support/src/external_deps/rustc.rs

@@ -52,13 +52,20 @@ impl Rustc {
     // `rustc` invocation constructor methods
 
     /// Construct a new `rustc` invocation. This will automatically set the library
-    /// search path as `-L cwd()` and also the compilation target.
+    /// search path as `-L cwd()`, configure the compilation target and enable
+    /// dynamic linkage by default on musl hosts.
     /// Use [`bare_rustc`] to avoid this.
     #[track_caller]
     pub fn new() -> Self {
         let mut cmd = setup_common();
         cmd.arg("-L").arg(cwd());
 
+        // FIXME: On musl hosts, we currently default to static linkage, while
+        // for running run-make tests, we rely on dynamic linkage by default
+        if std::env::var("IS_MUSL_HOST").is_ok_and(|i| i == "1") {
+            cmd.arg("-Ctarget-feature=-crt-static");
+        }
+
         // Automatically default to cross-compilation
         Self { cmd, target: Some(target()) }
     }