Unrolled build for rust-lang#138623

Rollup merge of rust-lang#138623 - daltenty:daltenty/fix-compiler-rt, r=Kobzol

[bootstrap] Use llvm_runtimes for compiler-rt

Trying to enable `compiler-rt` via `LLVM_ENABLE_PROJECTS` is no longer a supported option in LLVM, and gives you nasty warnings:
```
Using LLVM_ENABLE_PROJECTS=compiler-rt is deprecated now, and will become a
  fatal error in the LLVM 21 release.  Please use
  -DLLVM_ENABLE_RUNTIMES=compiler-rt or see the instructions at
  https://compiler-rt.llvm.org/ for building the runtimes.
```

try-job: aarch64-gnu-debug
try-job: x86_64-gnu-debug

Author: rust-timer

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

@@ -444,7 +444,6 @@ impl Step for Llvm {
 
         if helpers::forcing_clang_based_tests() {
             enabled_llvm_projects.push("clang");
-            enabled_llvm_projects.push("compiler-rt");
         }
 
         if builder.config.llvm_polly {
@@ -467,6 +466,10 @@ impl Step for Llvm {
 
         let mut enabled_llvm_runtimes = Vec::new();
 
+        if helpers::forcing_clang_based_tests() {
+            enabled_llvm_runtimes.push("compiler-rt");
+        }
+
         if builder.config.llvm_offload {
             enabled_llvm_runtimes.push("offload");
             //FIXME(ZuseZ4): LLVM intends to drop the offload dependency on openmp.