[compiler-rt] [compiler-rt][builtins] Add missing flag for builtins standalone build (PR #133046)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve2-vla-2stage` running on `linaro-g4-02` while building `compiler-rt` at step 12 "ninja check 2".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/199/builds/2422

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 12 (ninja check 2) failure: stage 2 checked (failure)
...
PASS: Flang :: Driver/print-resource-dir.F90 (24688 of 96338)
PASS: Flang :: Driver/predefined-macros-x86.f90 (24689 of 96338)
PASS: Flang :: Driver/predefined-macros-compiler-version.F90 (24690 of 96338)
PASS: Flang :: Driver/phases.f90 (24691 of 96338)
PASS: Flang :: Driver/include-header.f90 (24692 of 96338)
PASS: Flang :: Driver/missing-arg.f90 (24693 of 96338)
PASS: Flang :: Driver/mlir-pass-pipeline.f90 (24694 of 96338)
PASS: Flang :: Driver/parse-error.ll (24695 of 96338)
PASS: Flang :: Driver/parse-fir-error.ll (24696 of 96338)
UNRESOLVED: Flang :: Driver/slp-vectorize.ll (24697 of 96338)
******************** TEST 'Flang :: Driver/slp-vectorize.ll' FAILED ********************
Test has no 'RUN:' line
********************
PASS: Flang :: Driver/parse-ir-error.f95 (24698 of 96338)
PASS: Flang :: Driver/pass-plugin-not-found.f90 (24699 of 96338)
PASS: Flang :: Driver/print-pipeline-passes.f90 (24700 of 96338)
PASS: Flang :: Driver/multiple-input-files.f90 (24701 of 96338)
PASS: Flang :: Driver/scanning-error.f95 (24702 of 96338)
PASS: Flang :: Driver/print-target-triple.f90 (24703 of 96338)
PASS: Flang :: Driver/std2018-wrong.f90 (24704 of 96338)
PASS: Flang :: Driver/lto-flags.f90 (24705 of 96338)
PASS: Flang :: Driver/pthread.f90 (24706 of 96338)
PASS: Flang :: Driver/falias-analysis.f90 (24707 of 96338)
PASS: Flang :: Driver/supported-suffices/f03-suffix.f03 (24708 of 96338)
PASS: Flang :: Driver/input-from-stdin/input-from-stdin.f90 (24709 of 96338)
PASS: Flang :: Driver/supported-suffices/f08-suffix.f08 (24710 of 96338)
PASS: Flang :: Driver/lto-bc.f90 (24711 of 96338)
PASS: Flang :: Driver/tco-code-gen-llvm.fir (24712 of 96338)
PASS: Flang :: Driver/target-gpu-features.f90 (24713 of 96338)
PASS: Flang :: Driver/pp-fixed-form.f90 (24714 of 96338)
PASS: Flang :: Driver/target.f90 (24715 of 96338)
PASS: Flang :: Driver/mllvm.f90 (24716 of 96338)
PASS: Flang :: Driver/q-unused-arguments.f90 (24717 of 96338)
PASS: Flang :: Driver/fsave-optimization-record.f90 (24718 of 96338)
PASS: Flang :: Driver/unsupported-vscale-max-min.f90 (24719 of 96338)
PASS: Flang :: Driver/optimization-remark-invalid.f90 (24720 of 96338)
PASS: Flang :: Driver/unparse-with-modules.f90 (24721 of 96338)
PASS: Flang :: Driver/mlink-builtin-bc.f90 (24722 of 96338)
PASS: Flang :: Driver/no-duplicate-main.f90 (24723 of 96338)
PASS: Flang :: Driver/prescanner-diag.f90 (24724 of 96338)
PASS: Flang :: Driver/target-machine-error.f90 (24725 of 96338)
PASS: Flang :: Driver/response-file.f90 (24726 of 96338)
PASS: Flang :: Driver/unparse-use-analyzed.f95 (24727 of 96338)
PASS: Flang :: Driver/std2018.f90 (24728 of 96338)
PASS: Flang :: Driver/fveclib.f90 (24729 of 96338)
PASS: Clangd Unit Tests :: ./ClangdTests/73/81 (24730 of 96338)
PASS: Flang :: Driver/save-temps.f90 (24731 of 96338)
PASS: Flang :: Driver/optimization-remark-backend.f90 (24732 of 96338)
PASS: DataFlowSanitizer-aarch64 :: pair.cpp (24733 of 96338)

```

</details>

https://github.com/llvm/llvm-project/pull/133046