[clang] Link libclangBasic against libatomic when necessary

This may be necessary at least on PPC32.

https://bugs.gentoo.org/874024

--- /clang/CMakeLists.txt
+++ /clang/CMakeLists.txt
@@ -117,6 +117,7 @@
   include(TableGen)
   include(HandleLLVMOptions)
   include(VersionFromVCS)
+  include(CheckAtomic)
   include(GetErrcMessages)
   include(LLVMDistributionSupport)
 
--- /clang/lib/Basic/CMakeLists.txt
+++ /clang/lib/Basic/CMakeLists.txt
@@ -110,3 +110,7 @@
   omp_gen
   )
 
+target_link_libraries(clangBasic
+  PRIVATE
+  ${LLVM_ATOMIC_LIB}
+)