[llvm] Don't require third-party components when cross-compiling

It is possible to build LLVM with just the "llvm" and "cmake" components
checked out. This requires disabling the `LLVM_INCLUDE_BENCHMARKS` and
`LLVM_INCLUDE_TESTS` options. These options are not passed through to
the native build when cross-compiling though, so the build will break if
the "third-party" component is missing. We don't need the benchmarks or
tests for the native build, so disable these unconditionally. This fixes
cross-compiling on Gentoo, where only the required components are
checked out.

Differential Revision: https://reviews.llvm.org/D141196

llvm/cmake/modules/CrossCompile.cmake

@@ -84,6 +84,8 @@ function(llvm_create_cross_target project_name target_name toolchain buildtype)
         -DLLVM_ENABLE_RUNTIMES="${llvm_enable_runtimes_arg}"
         ${external_project_source_dirs}
         -DLLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN="${LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN}"
+        -DLLVM_INCLUDE_BENCHMARKS=OFF
+        -DLLVM_INCLUDE_TESTS=OFF
         ${build_type_flags} ${linker_flag} ${external_clang_dir}
         ${ARGN}
     WORKING_DIRECTORY ${${project_name}_${target_name}_BUILD}