[compiler-rt][builtins] Add missing flag for builtins standalone build

[yingcong-wu]

When builtins are built with runtimes, it is built before compiler-rt, and this makes some of the HAS_XXX_FLAGs missing. In this case, the COMPILER_RT_HAS_FCF_PROTECTION_FLAG is missing which makes it impossible to enable CET in this case. This patch addresses this issue by also check for such flag in standalone build instead of relying on the compiler-rt's detection.

[DavidTruby]

LGTM, seems a simple enough fix. Thanks!

[llvm-ci]

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

Here is the relevant piece of the build log for the reference

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)