Structure element mapping for OpenMP 5.0 v3

Structure element mapping for OpenMP 5.0 v3

This is a merge of patch:
https://gcc.gnu.org/pipermail/gcc-patches/2021-May/571515.html

v2 patch already merged at 18dd4f283e15894a26c9a105c4f87d9a585f93c5,
this commit only consists of the v2-to-v3 diff part in above URL.

This v3 adds a small bug fix, where the initialization of the refcount didn't
handle all cases, fixed by using gomp_refcount_increment here (more consistent).

libgomp/ChangeLog:

	* target.c (gomp_map_vars_internal): For new key entries, set
	k->refcount to 0, remove initialization of k->structelem_refcount,
	use gomp_increment_refcount to consistently handle all increment cases.

(manual cherry pick of e7073707bab79ceceaa0e2d25a632d03ac98d0fd)