[CMake] Fix runtimes build for host Windows (default target)

[krisb]

Summary

When building runtimes, the compiler name (e.g. clang, clang-cl) is set based on
CMAKE_SYSTEM_NAME passed to llvm_ExternalProject_Add() through CMAKE_ARGS argument.
This mechanism doesn't work well if the target is Windows host.
runtime_default_target()/builtin_default_target() doesn't provide a way
to specify CMAKE_SYSTEM_NAME and doesn't set it either.

This patch appends variables specified in RUNTIMES_CMAKE_ARGS/BUILTINS_CMAKE_ARGS
to CMAKE_ARGS argument of llvm_ExternalProject_Add() in the case of called
from runtime_default_target()/builtin_default_target() thus in particular
it allows passing CMAKE_SYSTEM_NAME whenever it is required.

As far as I can see from D73811 default target doesn't necessarily assume host.
So, this solution might be safer than D73811.

There is an alternative approach for this problem at D80866 (which looks
more vague and unclear).

[compnerd]

I definitely like this approach better. I would like to get @plotfi to sign off on this, but LGTM.

[plotfi]

LGTM

[phosek]

LGTM, I like this, thanks! I think we should use this approach for other flags as well, e.g. RUNTIMES_LLVM_ENABLE_RUNTIMES.

[krisb]

@plotfi @compnerd @phosek Thank you all for reviewing this!

llvm/runtimes/CMakeLists.txt

llvm_ExternalProject_Add(builtins
                              CMAKE_ARGS -DLLVM_LIBRARY_OUTPUT_INTDIR=${LLVM_LIBRARY_DIR}
                                         -DLLVM_RUNTIME_OUTPUT_INTDIR=${LLVM_TOOLS_BINARY_DIR}
                                         -DLLVM_DEFAULT_TARGET_TRIPLE=${TARGET_TRIPLE}
                                         -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON
                                         -DCMAKE_C_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_ASM_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_C_COMPILER_WORKS=ON
                                         -DCMAKE_ASM_COMPILER_WORKS=ON
+                                        ${BUILTINS_CMAKE_ARGS}
                              PASSTHROUGH_PREFIXES COMPILER_RT
                              USE_TOOLCHAIN
                              ${EXTRA_ARGS})
   endfunction()
 
   function(builtin_register_target compiler_rt_path target)
     cmake_parse_arguments(ARG "" "" "DEPENDS" ${ARGN})
 
llvm_ExternalProject_Add(runtimes
                                         -DLLVM_ENABLE_PROJECTS_USED=${LLVM_ENABLE_PROJECTS_USED}
                                         -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON
                                         -DCMAKE_C_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_CXX_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_ASM_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_C_COMPILER_WORKS=ON
                                         -DCMAKE_CXX_COMPILER_WORKS=ON
                                         -DCMAKE_ASM_COMPILER_WORKS=ON
+                                        ${RUNTIMES_CMAKE_ARGS}
                              PASSTHROUGH_PREFIXES LLVM_ENABLE_RUNTIMES
                                                   ${ARG_PREFIXES}
                              EXTRA_TARGETS ${extra_targets}
                                            ${test_targets}
                                            ${SUB_COMPONENTS}
                                            ${SUB_CHECK_TARGETS}
                                            ${SUB_INSTALL_TARGETS}
                              USE_TOOLCHAIN