CMAKE: help CheckAtomic find check_library_exists

lldb fails to build standalone on x86

-- Performing Test HAVE_CXX_ATOMICS64_WITHOUT_LIB - Failed
CMake Error at CheckAtomic.cmake:66 (check_library_exists):
  Unknown CMake command "check_library_exists".
Call Stack (most recent call first):
  LLDBStandalone.cmake:90 (include)
CMakeLists.txt:3 (include)

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D40220

llvm-svn: 319610