Fix Linux test.

Author: Greg Parker

unittests/runtime/LongTests/CMakeLists.txt

@@ -16,6 +16,11 @@ if(("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "${SWIFT_PRIMARY_VARIANT_SDK}") AND
 #      ${FOUNDATION_LIBRARY}
 #      swiftStdlibUnittest${SWIFT_PRIMARY_VARIANT_SUFFIX}
 #      )
+  elseif(SWIFT_HOST_VARIANT STREQUAL "linux")
+    find_library(ATOMIC_LIBRARY atomic)
+    list(APPEND PLATFORM_TARGET_LINK_LIBRARIES
+      ${ATOMIC_LIBRARY}
+      )
   elseif(SWIFT_HOST_VARIANT STREQUAL "freebsd")
     find_library(EXECINFO_LIBRARY execinfo)
     list(APPEND PLATFORM_TARGET_LINK_LIBRARIES