Spin 'dg-do run' part of 'libgomp.c/declare-variant-3-sm30.c' off into new 'libgomp...

Spin 'dg-do run' part of 'libgomp.c/declare-variant-3-sm30.c' off into new 'libgomp.c/declare-variant-3.c'

Having nvptx offloading configured doesn't imply being able to run nvptx
offloading test cases on the test host.

Also, make 'libgomp.c/declare-variant-3.c' work for all non-offloading and
offloading cases.

Fix-up for commit 59b8ade88774b4dcf1691a8f650cdbb86cc30862
"[libgomp, testsuite, nvptx] Add libgomp.c/declare-variant-3-sm*.c".

	libgomp/
	* testsuite/libgomp.c/declare-variant-3-sm30.c: Turn 'dg-do run'
	into 'dg-do link'.
	* testsuite/libgomp.c/declare-variant-3.c: New.
	* testsuite/libgomp.c/declare-variant-3.h: Extend.