* ggc.h (empty_string): Delete.
	* cfgexpand.c (expand_asm_stmt): Use plain "".
	* optabs.c (expand_asm_memory_barrier): Likewise.
	* stringpool.c (empty_string): Delete.
	(digit_vector, digit_string): Delete.
	(ggc_alloc_string): Use plain "", don't optimize single digit
	strings.  Use ggc_alloc_atomic.

From-SVN: r249851