Implement -Wimplicit-fallthrough: the rest

[Joseph Myers]

On Fri, 22 Jul 2016, Marek Polacek wrote:

> gcc/go/
> 	* go-system.h (go_fallthrough): Define.
> 	* gofrontend/escape.cc (Escape_analysis_assign::assign): Add
> 	go_fallthrough.
> 	* gofrontend/expressions.cc (Binary_expression::do_get_backend):
> 	Likewise.

gofrontend is externally maintained and should not be modified locally.

> libgcc/
> 	* soft-fp/op-common.h: Add _FP_FALLTHRU.
> 	* soft-fp/soft-fp.h (_FP_FALLTHRU): Define.

Likewise soft-fp (and I think a comment would be better than anything 
needing version conditionals).

-- 
Joseph S. Myers
joseph@codesourcery.com