[fortran] add __builtin_unreachable

[Tobias Burnus]

Jan Hubicka wrote:
> this patch add __buliltin_unreachable to Fortran FE and also cleans up the code a bit
>
> Bootstrapped/regtested x86_64-linux, OK?

The Fortran part looks okay.

> 	* f95-lang.c (gfc_define_builtin): Use set_call_expr_flags.
> 	(ATTR_NOTHROW_LEAF_MALLOC_LIST,
> 	ATTR_CONST_NORETURN_NOTHROW_LEAF_LIST): New lists.
> 	(gfc_init_builtin_functions): Add __builtin_unreachable;
> 	tidy malloc/calloc.
>
> 	* calls.c (set_call_expr_flags): New.
> 	* tree.h (set_call_expr_flags): Declare.

Tobias