re PR libgomp/79876 (FAIL: libgomp.fortran/strassen.f90 -O execution test on x86_6...

re PR libgomp/79876 (FAIL: libgomp.fortran/strassen.f90   -O  execution test on x86_64-apple-darwin16)

	PR libgomp/79876
	* config/posix/thread-stacksize.h: New file.
	* config/darwin/thread-stacksize.h: New file.
	* config/nvptx/thread-stacksize.h: New file.
	* env.c: Include thread-stacksize.h.
	(initialize_env): Initialize stacksize to GOMP_DEFAULT_STACKSIZE
	instead of 0.  Call pthread_attr_setstacksize even if
	GOMP_DEFAULT_STACKSIZE is non-zero.

From-SVN: r246675