support runtime LSE atomics detection for aarch64

Outline-atomics flag modifies builtin atomics to detect CPU support for
atomic instructions in runtime, thus getting menefits of using LSE
atomics without hurting compatibility.
This flag has no effect if compiling for aarch64 >=v8.1.

CMakeLists.txt

@@ -866,6 +866,15 @@ ELSE()
   OPTION(USE_LD_GOLD "Use GNU gold linker" OFF)
 ENDIF()
 
+#for aarch64 - allow dynamic use of LSE atomics
+IF(CMAKE_SYSTEM_PROCESSOR STREQUAL "aarch64")
+  MY_CHECK_CXX_COMPILER_FLAG( "-moutline-atomics" HAVE_OUTLINE_ATOMICS)
+  IF(HAVE_OUTLINE_ATOMICS)
+     STRING_APPEND(CMAKE_C_FLAGS   " -moutline-atomics")
+     STRING_APPEND(CMAKE_CXX_FLAGS " -moutline-atomics")
+  ENDIF()
+ENDIF()
+
 IF(LINUX)
   OPTION(LINK_RANDOMIZE "Randomize the order of all symbols in the binary" OFF)
   SET(LINK_RANDOMIZE_SEED "mysql"