Fix for bug #37778: _MemoryBarrier() in cpufunc.h error on compile

* include/avr/cpufunc.h: Place empty string for asm body. This is
needed for older GCC versions.
* tests/simulate/regression/bug-37778.c: New file.
* NEWS: Add to fixed bug list.



git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2307 1084037a-409f-4a99-8160-1a4b3577d4c1