[zorg] Add flang to clang-arm64-windows-msvc

[rovka]

Summary

This requires a small hack to add the path to clang's builtins
library to the link commands. We're putting that in an environment
variable so we can update it on the bot whenever we're updating
the clang version that we use.

[rovka]

Add comment about BUILTINS env variable.

[maxim-kuvyrkov]

Looks good to me. Thanks!

buildbot/osuosl/master/config/builders.py

 # Clang builders.
 
     {'name': "clang-arm64-windows-msvc",
     'tags' : ["clang"],
     'workernames' : ["linaro-armv8-windows-msvc-01", "linaro-armv8-windows-msvc-02"],
     'builddir': "clang-arm64-windows-msvc",
     'factory' : ClangBuilder.getClangCMakeBuildFactory(
                     vs="manual",
+                    checkout_flang=True,
                     test=False, # Disable testing until MCJIT failures are fixed
                     extra_cmake_args=[
                         "-DLLVM_DEFAULT_TARGET_TRIPLE=aarch64-windows-msvc",
                         "-DLLVM_HOST_TRIPLE=aarch64-windows-msvc",
                         "-DLLVM_TARGET_ARCH=AArch64",
+                        # The BUILTINS environment variable is expected to already exist
+                        # on the worker when it is launched.
+                        "-DCMAKE_EXE_LINKER_FLAGS=%BUILTINS%",
+                        "-DCMAKE_SHARED_LINKER_FLAGS=%BUILTINS%",
+                        "-DCMAKE_STATIC_LINKER_FLAGS=%BUILTINS%",
+                        "-DCMAKE_MODULE_LINKER_FLAGS=%BUILTINS%",
                         # FIXME: compiler-rt\lib\sanitizer_common\sanitizer_unwind_win.cpp assumes WIN64 is x86_64,
                         #        so, before that's fixed, disable everything that triggers its build.
                         "-DCOMPILER_RT_BUILD_SANITIZERS=OFF",
                         "-DCOMPILER_RT_BUILD_MEMPROF=OFF",
                         "-DCOMPILER_RT_BUILD_XRAY=OFF"])},
 
     # Cortex-A15 LNT test-suite in Benchmark mode
     {'name' : "clang-native-arm-lnt-perf",