configure.target (cpu_include_dir): Set to `config/cpu/m68k' for m68k and m680[246]0.

	* configure.target (cpu_include_dir): Set to `config/cpu/m68k' for
	m68k and m680[246]0.
	* config/cpu/m68k/bits/atomicity.h: New file.

From-SVN: r45006

libstdc++-v3/configure.target

@@ -39,6 +39,9 @@ case "${target_cpu}" in
   i486 | i586 | i686 | i786)
     cpu_include_dir="config/cpu/i486"
     ;;
+  m68k | m680[246]0)
+    cpu_include_dir="config/cpu/m68k"
+    ;;
   powerpc | rs6000)
     cpu_include_dir="config/cpu/powerpc"
     ;;