Add a new control variable to the Gogo class that stores the size
    threshold for nil checks. This value can be used to control the
    policy for deciding when a given deference operation needs a check and
    when it does not. A size threshold of -1 means that every potentially
    faulting dereference needs an explicit check (and branch to error
    call). A size threshold of K (where K > 0) means that if the size of
    the object being dereferenced is >= K, then we need a check.
    
    Reviewed-on: https://go-review.googlesource.com/80996

	* go-c.h (go_create_gogo_args): Add nil_check_size_threshold
	field.
	* go-lang.c (go_langhook_init): Set nil_check_size_threshold.

From-SVN: r255340