lld: link libatomic if needed for Timer

D80298 made Timer::total atomic, but this requires linking libatomic
on some targets.

This has been submitted as D85691 to fix our arm-unknown-linux-gnueabi
build, but there's concern about targets that don't even have libatomic.

See also: https://bugs.llvm.org/show_bug.cgi?id=47123

Author: cuviper

lld/CMakeLists.txt

@@ -54,6 +54,7 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   include(AddLLVM)
   include(TableGen)
   include(HandleLLVMOptions)
+  include(CheckAtomic)
 
   if(LLVM_INCLUDE_TESTS)
     if(CMAKE_VERSION VERSION_LESS 3.12)

lld/Common/CMakeLists.txt

@@ -1,3 +1,10 @@
+set(LLD_SYSTEM_LIBS ${LLVM_PTHREAD_LIB})
+
+# See also: https://bugs.llvm.org/show_bug.cgi?id=47123
+if(NOT HAVE_CXX_ATOMICS64_WITHOUT_LIB)
+  list(APPEND LLD_SYSTEM_LIBS atomic)
+endif()
+
 if(NOT LLD_BUILT_STANDALONE)
   set(tablegen_deps intrinsics_gen)
 endif()
@@ -54,7 +61,7 @@ add_lld_library(lldCommon
   Target
 
   LINK_LIBS
-  ${LLVM_PTHREAD_LIB}
+  ${LLD_SYSTEM_LIBS}
 
   DEPENDS
   ${tablegen_deps}