Add ability to use a CMake toolchains file for specifying details of the stage1 CC/CXX

This allows us to follow best/usual CMake practice for configuring cross
compiling.

Author: asb

zorg/buildbot/builders/ClangBuilder.py

@@ -162,6 +162,10 @@ def getClangCMakeBuildFactory(
             # CMake arguments to use for stage2 instead of extra_cmake_args.
             extra_stage2_cmake_args=None,
 
+            # If set, use a toolchains file for the stage 2 build and add
+            # these options to it.
+            stage2_toolchain_options=None,
+
             # Extra repositories
             checkout_clang_tools_extra=True,
             checkout_compiler_rt=True,
@@ -180,6 +184,7 @@ def getClangCMakeBuildFactory(
                submitURL=submitURL, testerName=testerName,
                env=env, extra_cmake_args=extra_cmake_args,
                extra_stage2_cmake_args=extra_stage2_cmake_args,
+               stage2_toolchain_options=stage2_toolchain_options,
                checkout_clang_tools_extra=checkout_clang_tools_extra,
                checkout_lld=checkout_lld,
                checkout_compiler_rt=checkout_compiler_rt,
@@ -219,6 +224,10 @@ def _getClangCMakeBuildFactory(
             # CMake arguments to use for stage2 instead of extra_cmake_args.
             extra_stage2_cmake_args=None,
 
+            # If set, use a toolchains file for the stage 2 build and add
+            # these options to it.
+            stage2_toolchain_options=None,
+
             # Extra repositories
             checkout_clang_tools_extra=True,
             checkout_compiler_rt=True,
@@ -427,9 +436,31 @@ def _getClangCMakeBuildFactory(
         # Note: Backslash path separators do not work well with cmake and ninja.
         # Forward slash path separator works on Windows as well.
         stage1_cc = InterpolateToPosixPath(
-                        f"-DCMAKE_C_COMPILER=%(prop:builddir)s/{stage1_install}/bin/{cc}")
+                        f"%(prop:builddir)s/{stage1_install}/bin/{cc}")
         stage1_cxx = InterpolateToPosixPath(
-                        f"-DCMAKE_CXX_COMPILER=%(prop:builddir)s/{stage1_install}/bin/{cxx}")
+                        f"%(prop:builddir)s/{stage1_install}/bin/{cxx}")
+
+        # If stage2_toolchain_options is set when we'll use a toolchain file
+        # to specify the compiler being used (the just-built stage1) and add
+        # any stage2_toolchain_options to it. Otherwise, just set
+        # -DCMAKE_{C,CXX}_COMPILER.
+        if stage2_toolchain_options is None:
+            compiler_args = [
+                    f"-DCMAKE_C_COMPILER={stage1_cc}",
+                    f"-DCMAKE_CXX_COMPILER={stage1_cxx}"
+            ]
+        else:
+            toolchain_file = InterpolateToPosixPath(
+                        f"%(prop:builddir)s/{stage2_build}/stage1-toolchain.cmake")
+            with open(toolchain_file, 'w') as file:
+                file.write(f"set(CMAKE_C_COMPILER {stage1_cc})\n")
+                file.write(f"set(CMAKE_CXX_COMPILER {stage1_cxx})\n")
+                for option in stage2_toolchain_options:
+                    file.write(f"{option}\n")
+
+            compiler_args = [
+                    "-DCMAKE_TOOLCHAIN_FILE={toolchain_file}"
+                    ]
 
         # If we have a separate stage2 cmake arg list, then ensure we re-apply
         # enable_projects and enable_runtimes if necessary.
@@ -445,13 +476,12 @@ def _getClangCMakeBuildFactory(
 
         rel_src_dir = LLVMBuildFactory.pathRelativeTo(f.llvm_srcdir, stage2_build)
         cmake_cmd2 = [cmake, "-G", "Ninja", rel_src_dir,
-                      stage1_cc,
-                      stage1_cxx,
                       f"-DCMAKE_BUILD_TYPE={stage2_config}",
                       "-DLLVM_ENABLE_ASSERTIONS=True",
                       f"-DLLVM_LIT_ARGS={lit_args}",
                       f"-DCMAKE_INSTALL_PREFIX=../{stage2_install}"
-                     ] + (extra_stage2_cmake_args or extra_cmake_args)
+                     ] + (extra_stage2_cmake_args or extra_cmake_args) \
+                       + compiler_args
 
         f.addStep(ShellCommand(name='cmake stage 2',
                                command=cmake_cmd2,