[llvm] [Release] Install compiler-rt builtins during Phase 1 on AIX (PR #81485)

[Amy Kwan via llvm-commits]

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@@ -534,7 +534,7 @@ function build_llvmCore() {
       # compiler-rt builtins is needed on AIX to have a functional Phase 1 clang.
       if [ "$System" = "AIX" -o "$Phase" != "1" ]; then
         BuildTarget="$BuildTarget runtimes"
-        InstallTarget="$InstallTarget install-runtimes"
+        InstallTarget="$InstallTarget install-runtimes install-builtins"
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amy-kwan wrote:

@tstellar Just wanted to check, but in the original [commit](https://github.com/llvm/llvm-project/pull/72703/commits/fcae8ce77e2256920459fb4cf428a212d6560b68) that added this section, there's no explicit `install-builtins` so did we just miss adding it or is its absence intentional?

We realized that `install-runtimes` does not actually install the compiler-rt builtins, so it appears that `install-builtins` is needed here. 

Also, it would seem that this section can run on non-AIX systems, and on Phase 2, and I'm not quite sure if that matches the comment on 534. Is this also intentional?

https://github.com/llvm/llvm-project/pull/81485