In 'libgomp.c/declare-variant-{3,4}-*.c', restrict 'scan-offload-tree-dump's to ...

In 'libgomp.c/declare-variant-{3,4}-*.c', restrict 'scan-offload-tree-dump's to 'only_for_offload_target [...]'

... to care for the case where not just one but both of GCN and nvptx
offloading are enabled.  In that case, we currently get:

    UNRESOLVED: libgomp.c/declare-variant-3-sm30.c scan-amdgcn-amdhsa-offload-tree-dump optimized "= f30 \\(\\);"

... in addition to:

    PASS: libgomp.c/declare-variant-3-sm30.c scan-nvptx-none-offload-tree-dump optimized "= f30 \\(\\);"

Etc.

Fix-up for commit 59b8ade88774b4dcf1691a8f650cdbb86cc30862
"[libgomp, testsuite, nvptx] Add libgomp.c/declare-variant-3-sm*.c",
and commit 1fd508744eccda9ad9c6d6fcce5b2ea9c568818d
"amdgcn: Support AMD-specific 'isa' traits in OpenMP context selectors".

	libgomp/
	* testsuite/libgomp.c/declare-variant-3-sm30.c: Restrict
	'scan-offload-tree-dump' to 'only_for_offload_target nvptx-none'.
	* testsuite/libgomp.c/declare-variant-3-sm35.c: Likewise.
	* testsuite/libgomp.c/declare-variant-3-sm53.c: Likewise.
	* testsuite/libgomp.c/declare-variant-3-sm70.c: Likewise.
	* testsuite/libgomp.c/declare-variant-3-sm75.c: Likewise.
	* testsuite/libgomp.c/declare-variant-3-sm80.c: Likewise.
	* testsuite/libgomp.c/declare-variant-4-fiji.c: Restrict
	'scan-offload-tree-dump' to
	'only_for_offload_target amdgcn-amdhsa'.
	* testsuite/libgomp.c/declare-variant-4-gfx803.c: Likewise.
	* testsuite/libgomp.c/declare-variant-4-gfx900.c: Likewise.
	* testsuite/libgomp.c/declare-variant-4-gfx906.c: Likewise.
	* testsuite/libgomp.c/declare-variant-4-gfx908.c: Likewise.
	* testsuite/libgomp.c/declare-variant-4-gfx90a.c: Likewise.