[CMake][libcxx] Fix the --target and --gcc-toolchain flag handling

[phosek]

Summary

CMake has the problem with the single dash variant because of the space, so use the double dash with equal sign version. We also don't have to pass the target triple when checking for compiler-rt since that flag is already included in compile flags now.

[phosek]

Ping, can someone please take a look?

[phosek]

Thanks, can you also please take a look at D32069 which is the same change for libc++abi?

libcxx/trunk/CMakeLists.txt

 # Include macros for adding and removing libc++ flags.
 include(HandleLibcxxFlags)
 
 # Target flags ================================================================
 # These flags get added to CMAKE_CXX_FLAGS and CMAKE_C_FLAGS so that
 # 'config-ix' use them during feature checks. It also adds them to both
 # 'LIBCXX_COMPILE_FLAGS' and 'LIBCXX_LINK_FLAGS'
 add_target_flags_if(LIBCXX_BUILD_32_BITS "-m32")
-add_target_flags_if(LIBCXX_TARGET_TRIPLE "-target ${LIBCXX_TARGET_TRIPLE}")
+add_target_flags_if(LIBCXX_TARGET_TRIPLE "--target=${LIBCXX_TARGET_TRIPLE}")
 add_target_flags_if(LIBCXX_SYSROOT "--sysroot=${LIBCXX_SYSROOT}")
-add_target_flags_if(LIBCXX_GCC_TOOLCHAIN "-gcc-toolchain ${LIBCXX_GCC_TOOLCHAIN}")
+add_target_flags_if(LIBCXX_GCC_TOOLCHAIN "--gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
 if (LIBCXX_TARGET_TRIPLE)
   set(TARGET_TRIPLE "${LIBCXX_TARGET_TRIPLE}")
 endif()
 
 # Configure compiler.
 include(config-ix)
 
 if (LIBCXX_USE_COMPILER_RT)

libcxx/trunk/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} ${TARGET_TRIPLE} ${LIBCXX_COMPILE_FLAGS}
+  set(CLANG_COMMAND ${CMAKE_CXX_COMPILER} ${LIBCXX_COMPILE_FLAGS}
       "--rtlib=compiler-rt" "--print-libgcc-file-name")
   if (CMAKE_CXX_COMPILER_ID MATCHES Clang AND CMAKE_CXX_COMPILER_TARGET)
     list(APPEND CLANG_COMMAND "--target=${CMAKE_CXX_COMPILER_TARGET}")
   endif()
   execute_process(
       COMMAND ${CLANG_COMMAND}
       RESULT_VARIABLE HAD_ERROR
       OUTPUT_VARIABLE LIBRARY_FILE