[D] Don't predefine GNU_StackGrowsDown on every target.

[D] Don't predefine GNU_StackGrowsDown on every target.

Every back-end defines STACK_GROWS_DOWNWARD, the condition should
instead be  checking whether it was defined as 1 or 0.

gcc/d/ChangeLog:

2019-01-20  Iain Buclaw  <ibuclaw@gdcproject.org>

	* d-builtins.cc (d_init_versions): Check value of
	STACK_GROWS_DOWNWARD.

From-SVN: r268106