[compiler-rt] Don't explicitly set CMAKE_CXX_FLAGS.

Summary:
C++ flags should not be used for not-C++ files as it may trigger
-Werror=unused-command-line-argument. CMake will use CMAKE_C_FLAGS,
CMAKE_CXX_FLAGS, and CMAKE_ASM_FLAGS as appropriate implicitly, so
this does not need to be explicitly handled here.

This change depends on https://reviews.llvm.org/D53301, since one of
the builders depended on this behavior because it was not configuring
CMAKE_ASM_FLAGS.

Reviewers: eugenis, vitalybuka

Reviewed By: eugenis, vitalybuka

Subscribers: dberris, mgorny, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D53335

llvm-svn: 344751