LLVM_ENABLE_RUNTIMES=flang-rt for ClangBuilder builders (#386)

With the option `checkout_flang=True`, ClangBuilder-based builders also
compile Flang. Modify ClangBuilder to use
`LLVM_ENABLE_RUNTIMES=flang-rt`. This prepares the removal of the
"projects" build of the flang runtime in
llvm/llvm-project#124126.

Affected builders:
 * clang-aarch64-full-2stage
 * clang-aarch64-sve-vla
 * clang-aarch64-sve-vla-2stage
 * clang-aarch64-sve-vls
 * clang-aarch64-sve-vls-2stage
 * clang-aarch64-sve2-vla
 * clang-aarch64-sve2-vla-2stage
 * clang-arm64-windows-msvc
 * clang-arm64-windows-msvc-2stage

Affected workers:
 * linaro-clang-aarch64-full-2stage
 * linaro-g3-01
 * linaro-g3-02
 * linaro-g3-03
 * linaro-g3-04
 * linaro-g4-01
 * linaro-g4-02
 * linaro-armv8-windows-msvc-04
 * linaro-armv8-windows-msvc-02

zorg/buildbot/builders/ClangBuilder.py

@@ -286,6 +286,7 @@ def _getClangCMakeBuildFactory(
         depends_on_projects.append('libunwind')
     if checkout_flang:
         depends_on_projects.append('flang')
+        depends_on_projects.append('flang-rt')
         depends_on_projects.append('mlir')
 
     f = LLVMBuildFactory(