[Release] Build compiler-rt during Phase 1 on AIX (#70672)

Compiler-rt built-ins are needed to have a functional Phase 1 clang
compiler on AIX. This PR adds compiler-rt to the runtime_list during Phase 1 to avoid
this problem.

---------

Co-authored-by: Alison Zhang <alisonzhang@ibm.com>

llvm/utils/release/test-release.sh

@@ -417,6 +417,10 @@ function configure_llvmCore() {
     # building itself and any selected runtimes in the second phase.
     if [ "$Phase" -lt "2" ]; then
       runtime_list=""
+      # compiler-rt builtins is needed on AIX to have a functional Phase 1 clang.
+      if [ "$System" = "AIX" ]; then
+        runtime_list="compiler-rt"
+      fi  
     else
       runtime_list="$runtimes"
     fi