Fix Linux build, maybe.

Author: Greg Parker

cmake/modules/AddSwift.cmake

@@ -325,7 +325,7 @@ function(_add_variant_link_flags)
     RESULT_VAR_NAME result)
 
   if("${LFLAGS_SDK}" STREQUAL "LINUX")
-    list(APPEND result "-lpthread" "-ldl")
+    list(APPEND result "-lpthread" "-ldl" "-latomic")
   elseif("${LFLAGS_SDK}" STREQUAL "FREEBSD")
     list(APPEND result "-lpthread")
   elseif("${LFLAGS_SDK}" STREQUAL "CYGWIN")