bootstrap: only build crt{begin,end}.o when compiling to MUSL

only MUSL needs those objects and trying to compile them to other
targets, e.g. Windows or macOS, will produce C compilation errors

check the target before shelling out to the C compiler and tweak
`make_run` to skip the actual C compilation when the target is not MUSL

fixes rust-lang#135782

src/bootstrap/src/core/build_steps/compile.rs

@@ -339,7 +339,7 @@ fn copy_self_contained_objects(
     // to using gcc from a glibc-targeting toolchain for linking.
     // To do that we have to distribute musl startup objects as a part of Rust toolchain
     // and link with them manually in the self-contained mode.
-    if target.contains("musl") && !target.contains("unikraft") {
+    if target.needs_crt_begin_end() {
         let srcdir = builder.musl_libdir(target).unwrap_or_else(|| {
             panic!("Target {:?} does not have a \"musl-libdir\" key", target.triple)
         });

src/bootstrap/src/core/build_steps/llvm.rs

@@ -1295,11 +1295,19 @@ impl Step for CrtBeginEnd {
     }
 
     fn make_run(run: RunConfig<'_>) {
-        run.builder.ensure(CrtBeginEnd { target: run.target });
+        if run.target.needs_crt_begin_end() {
+            run.builder.ensure(CrtBeginEnd { target: run.target });
+        }
     }
 
     /// Build crtbegin.o/crtend.o for musl target.
     fn run(self, builder: &Builder<'_>) -> Self::Output {
+        assert!(
+            self.target.needs_crt_begin_end(),
+            "tried to build crtbegin.o and crtend.o for the wrong target ({})",
+            self.target
+        );
+
         builder.require_submodule(
             "src/llvm-project",
             Some("The LLVM sources are required for the CRT from `compiler-rt`."),

src/bootstrap/src/core/config/config.rs

@@ -573,6 +573,10 @@ impl TargetSelection {
         env::var("OSTYPE").is_ok_and(|v| v.to_lowercase().contains("cygwin"))
     }
 
+    pub fn needs_crt_begin_end(&self) -> bool {
+        self.contains("musl") && !self.contains("unikraft")
+    }
+
     /// Path to the file defining the custom target, if any.
     pub fn filepath(&self) -> Option<&Path> {
         self.file.as_ref().map(Path::new)