[Release] Build compiler-rt during Phase 1 on AIX

[azhan92]

Compiler-rt built-ins are needed to have a functional Phase 1 clang compiler on AIX:

CMake Error at /gsa/rtpgsa-p2/00/xlcmpbldenv/aix/cmake-3.22.0/share/cmake-3.22/Modules/CMakeTestCCompiler.cmake:69 (message):
  The C compiler

    "/scratch/alisonz/llvm/clang17.0.3-final/final/Phase1/Release/llvmCore-17.0.3-final.install/usr/local/bin/clang"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: /scratch/alisonz/llvm/clang17.0.3-final/final/Phase2/Release/llvmCore-17.0.3-final.obj/CMakeFiles/CMakeTmp
    
    Run Build Command(s):/xl_le/gsa/rtpgsa/projects/w/wyvern-environment/compilers/AIX/ninja cmTC_6ad06 && [1/2] Building C object CMakeFiles/cmTC_6ad06.dir/testCCompiler.c.o
    [2/2] Linking C executable cmTC_6ad06
    FAILED: cmTC_6ad06 
    : && /scratch/alisonz/llvm/clang17.0.3-final/final/Phase1/Release/llvmCore-17.0.3-final.install/usr/local/bin/clang  -Wl,-bnoipath  CMakeFiles/cmTC_6ad06.dir/testCCompiler.c.o -o cmTC_6ad06  -Wl,-blibpath:/usr/lib:/lib && :
    ld: 0706-005 Cannot find or open file: /scratch/alisonz/llvm/clang17.0.3-final/final/Phase1/Release/llvmCore-17.0.3-final.install/usr/local/lib/clang/17/lib/aix/libclang_rt.builtins-powerpc64.a
        ld:open(): A file or directory in the path name does not exist.
    clang: error: linker command failed with exit code 255 (use -v to see invocation)
    ninja: build stopped: subcommand failed.
    

  CMake will not be able to correctly generate this project.
Call Stack (most recent call first):
  CMakeLists.txt:47 (project)


-- Configuring incomplete, errors occurred!

This PR adds compiler-rt to the runtime_list during Phase 1 to avoid this problem.

[daltenty]

LGTM, with minor nit

[amy-kwan]

LGTM.