OpenMP: Fix 'omp declare target' handling for vars [PR99509]

OpenMP: Fix 'omp declare target' handling for vars [PR99509]

For variables with 'declare target' attribute,
varpool_node::get_create marks variables as offload; however,
if the node already exists, it is not updated. C/C++ may tag
decl with 'declare target implicit', which may only be after
varpool creation turned into 'declare target' or 'declare target link';
in this case, the tagging has to happen in the FE.

gcc/c/ChangeLog:

	PR c++/99509
	* c-decl.c (finish_decl): For 'omp declare target implicit' vars,
	ensure that the varpool node is marked as offloadable.

gcc/cp/ChangeLog:

	PR c++/99509
	* decl.c (cp_finish_decl): For 'omp declare target implicit' vars,
	ensure that the varpool node is marked as offloadable.

libgomp/ChangeLog:

	PR c++/99509
	* testsuite/libgomp.c-c++-common/declare_target-1.c: New test.