-Wimplicit-fallthrough noreturn fixes

-Wimplicit-fallthrough noreturn fixes

binutils/
	* cxxfilt.c (usage): Add ATTRIBUTE_NORETURN.
	* elfedit.c (usage): Likewise.
	* nm.c (usage): Likewise.
	* objcopy.c (copy_usage, strip_usage): Likewise.
	* srconv.c (show_usage): Likewise.
	* strings.c (usage): Likewise.
	* sysdump.c (show_usage): Likewise.
	* srconv.c: Remove unneeded forward function declarations.
	* strings.c: Likewise.
	* sysdump.c: Likewise.
gas/
	* as.h (as_assert): Add ATTRIBUTE_NORETURN.