Revert "Introduce a cmake module to figure out whether we need to link with libatomic."

This reverts commit r260235. It breaks LLVM's bootstrap when building
with a -gcc-toolchain and the system's gcc installation does not provide
the libatomic library and its headers. We should check whether
LIBCXX_GCC_TOOLCHAIN is set and adjust the flags accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@260323 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/Modules/CheckLibcxxAtomic.cmake

@@ -1,38 +0,0 @@
-INCLUDE(CheckCXXSourceCompiles)
-
-# Sometimes linking against libatomic is required for atomic ops, if
-# the platform doesn't support lock-free atomics.
-#
-# We could modify LLVM's CheckAtomic module and have it check for 64-bit
-# atomics instead. However, we would like to avoid careless uses of 64-bit
-# atomics inside LLVM over time on 32-bit platforms.
-
-function(check_cxx_atomics varname)
-  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "-std=c++11")
-  check_cxx_source_compiles("
-#include <cstdint>
-#include <atomic>
-std::atomic<uintptr_t> x;
-std::atomic<uintmax_t> y;
-int main() {
-  return x + y;
-}
-" ${varname})
-  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
-endfunction(check_cxx_atomics)
-
-check_cxx_atomics(HAVE_CXX_ATOMICS_WITHOUT_LIB)
-# If not, check if the library exists, and atomics work with it.
-if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
-  check_library_exists(atomic __atomic_fetch_add_8 "" HAVE_LIBATOMIC)
-  if(HAVE_LIBATOMIC)
-    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
-    check_cxx_atomics(HAVE_CXX_ATOMICS_WITH_LIB)
-    if (NOT HAVE_CXX_ATOMICS_WITH_LIB)
-      message(FATAL_ERROR "Host compiler must support std::atomic!")
-    endif()
-  else()
-    message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
-  endif()
-endif()