[testsuite] add linkonly to dg-additional-sources [PR115295]

The D testsuite shows it was a mistake to assume that
dg-additional-sources are never to be used for compilation tests.
Even if an output file is specified for compilation, extra module
files can be named and used in the compilation without being flagged
as errors.

Introduce a 'linkonly' flag for dg-additional-sources, and use it in
pr95401.cc and other vector tests that default to run, so that its
additional sources get discarded when vector tests downgrade to
compile-only.  This reverts previous workarounds for this very
circumstance, that relied on being able to run vector tests anyway,
even after failing to detect runtime or hardware vector support.


for  gcc/ChangeLog

	PR d/115295
	* doc/sourcebuild.texi (dg-additional-sources): Add linkonly.

for  gcc/testsuite/ChangeLog

	PR d/115295
	* g++.dg/vect/pr95401.cc: Add linkonly to dg-additional-sources.
	* g++.dg/vect/pr68762-1.cc: Likewise.
	* g++.dg/vect/simd-clone-3.cc: Likewise.
	* g++.dg/vect/simd-clone-5.cc: Likewise.
	* gcc.dg/vect/vect-simd-clone-10.c: Likewise.  Drop dg-do run.
	* gcc.dg/vect/vect-simd-clone-12.c: Likewise.  Likewise.
	* lib/gcc-defs.exp (additional_sources_omit_on_compile): New.
	(dg-additional-sources): Add to it on linkonly.
	(dg-additional-files-options): Omit select sources on compile.