[CMake][libcxx] Use builtins rather than gcc_s when compiler-rt is requested

[phosek]

Summary

When compiler-rt is requested, we should attempt to link compiler-rt builtins library rather than gcc_s.

[beanz]

Looks reasonable to me.

[mgorny]

See also: D25568. It served the same goal, except that it didn't require the user to explicitly indicate which runtime is enabled in his compiler (i.e. supported clang with compiler-rt by default out of the box). I'm not sure if having both makes sense now; however, I feel like having to figure out whether the compiler is using compiler-rt or libgcc and setting flags appropriately would kinda suck.

[phosek]

I'd prefer not having to manually specify which runtime to use as well, but it seems like there're still some unresolved issues in D25568. This change is just a fix to ensure that the existing logic works in all cases. If D25568 gets approved and lands I'd be fine if it replaces the this logic.

cmake/Modules/HandleCompilerRT.cmake

 function(find_compiler_rt_library name dest)
   if (NOT DEFINED LIBCXX_COMPILE_FLAGS)
     message(FATAL_ERROR "LIBCXX_COMPILE_FLAGS must be defined when using this function")
   endif()
   set(dest "" PARENT_SCOPE)
-  set(CLANG_COMMAND ${CMAKE_CXX_COMPILER} ${LIBCXX_COMPILE_FLAGS}
+  set(CLANG_COMMAND ${CMAKE_CXX_COMPILER} ${TARGET_TRIPLE} ${LIBCXX_COMPILE_FLAGS}
       "--rtlib=compiler-rt" "--print-libgcc-file-name")
+  if (CMAKE_CXX_COMPILER_ID MATCHES Clang AND CMAKE_CXX_COMPILER_TARGET)
+    list(APPEND CLANG_COMMAND "--target=${CMAKE_CXX_COMPILER_TARGET}")
+  endif()
   execute_process(
       COMMAND ${CLANG_COMMAND}
       RESULT_VARIABLE HAD_ERROR
       OUTPUT_VARIABLE LIBRARY_FILE
   )
   string(STRIP "${LIBRARY_FILE}" LIBRARY_FILE)
   string(REPLACE "builtins" "${name}" LIBRARY_FILE "${LIBRARY_FILE}")
   if (NOT HAD_ERROR AND EXISTS "${LIBRARY_FILE}")

cmake/config-ix.cmake

 # configuration checks to avoid spurious link errors.
 check_cxx_compiler_flag(-nodefaultlibs LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
 if (LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
   if (LIBCXX_HAS_C_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
   if (LIBCXX_USE_COMPILER_RT)
-    list(APPEND CMAKE_REQUIRED_LIBRARIES -rtlib=compiler-rt)
+    list(APPEND CMAKE_REQUIRED_FLAGS -rtlib=compiler-rt)
+    find_compiler_rt_library(builtins LIBCXX_BUILTINS_LIBRARY)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBCXX_BUILTINS_LIBRARY}")
   elseif (LIBCXX_HAS_GCC_S_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES gcc_s)
   endif ()
   if (CMAKE_C_FLAGS MATCHES -fsanitize OR CMAKE_CXX_FLAGS MATCHES -fsanitize)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize=all")
   endif ()
   if (CMAKE_C_FLAGS MATCHES -fsanitize-coverage OR CMAKE_CXX_FLAGS MATCHES -fsanitize-coverage)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters")

lib/CMakeLists.txt

if (APPLE AND LLVM_USE_SANITIZER)
   endif()
 endif()
 
 # Generate private library list.
 add_library_flags_if(LIBCXX_HAS_PTHREAD_LIB pthread)
 add_library_flags_if(LIBCXX_HAS_C_LIB c)
 add_library_flags_if(LIBCXX_HAS_M_LIB m)
 add_library_flags_if(LIBCXX_HAS_RT_LIB rt)
+if (LIBCXX_USE_COMPILER_RT)
+  find_compiler_rt_library(builtins LIBCXX_BUILTINS_LIBRARY)
+  add_library_flags_if(LIBCXX_BUILTINS_LIBRARY "${LIBCXX_BUILTINS_LIBRARY}")
+else()
   add_library_flags_if(LIBCXX_HAS_GCC_S_LIB gcc_s)
+endif()
 add_library_flags_if(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB atomic)
 
 # Add the unwinder library.
 if (LIBCXXABI_USE_LLVM_UNWINDER)
   if (NOT LIBCXXABI_ENABLE_STATIC_UNWINDER AND (TARGET unwind_shared OR HAVE_LIBUNWIND))
     add_interface_library(unwind_shared)
   elseif (LIBCXXABI_ENABLE_STATIC_UNWINDER AND (TARGET unwind_static OR HAVE_LIBUNWIND))
     add_interface_library(unwind_static)