[cmake] [compiler-rt] Remove duplicate CMAKE_CXX_FLAGS.

[hintonda]

Summary

set_target_compile_flags() ultimately sets COMPILE_FLAGS which are
added to CMAKE_CXX_FLAGS in the compile rule, so passing
CMAKE_CXX_FLAGS causes them to be duplicated.

[phosek]

LGTM

[hintonda]

This patch caused http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-android/builds/7195 to fail and was reverted.

The problem seems to be that this buildbot invokes cmake with --target= and --sysroot= added to only CMAKE_CXX_FLAGS, and relies on set_target_flags to add them to COMPILE_FLAGS so it can be used for every compiler, e.g., C, CXX, and ASM. That's why the build broke when CMAKE_CXX_FLAGS was no longer added to COMPILE_FLAGS, i.e., the target= and --sysroot= flags weren't passed to ASM.

A better way to do this is to pass CMAKE_SYSROOT= and CMAKE_<lang>_COMPILER_TARGET=, which is how llvm/runtime/CMakeLists.txt does it.

Also, darwin_add_builtin_libraries() and darwin_add_embedded_builtin_libraries() currently reset CMAKE_<lang>_FLAGS, which means they must come from COMPILE_FLAGS.

[hintonda]

Not absolutely needed, and no time to work on this right now.

compiler-rt/trunk/cmake/Modules/AddCompilerRT.cmake

foreach(libname ${libnames})
 
     # Strip out -msse3 if this isn't macOS.
     set(target_flags ${LIB_CFLAGS})
     if(APPLE AND NOT "${libname}" MATCHES ".*\.osx.*")
       list(REMOVE_ITEM target_flags "-msse3")
     endif()
 
     set_target_compile_flags(${libname}
-      ${CMAKE_CXX_FLAGS} ${extra_cflags_${libname}} ${target_flags})
+      ${extra_cflags_${libname}} ${target_flags})
     set_property(TARGET ${libname} APPEND PROPERTY
       COMPILE_DEFINITIONS ${LIB_DEFS})
     set_target_properties(${libname} PROPERTIES FOLDER "Compiler-RT Libraries")
     if(APPLE)
       set_target_properties(${libname} PROPERTIES
         OSX_ARCHITECTURES "${LIB_ARCHS_${libname}}")
     endif()
   endforeach()