libgomp: Unbreak gcn offload build

libgomp: Unbreak gcn offload build

My recent libgomp change apparently broke libgomp build for gcn offloading.
The problem is that gcn, unlike nvptx, doesn't override teams.c source file
and the patch I've committed assumed all the non-LIBGOMP_USE_PTHREADS targets
do not use it.  My understanding is that gcn included omp_get_num_teams
and omp_get_team_num definitions in both icv-device.o and teams.o,
with the definitions only in the former working correctly.

This patch brings gcn into sync with how nvptx does it, that teams.c
is overridden, provides a dummy GOMP_teams_reg and omp_get_{num_teams,team_num}
definitions and icv-device.c doesn't provide those.

2021-11-12  Jakub Jelinek  <jakub@redhat.com>

	PR target/103201
	* config/gcn/icv-device.c (omp_get_num_teams, omp_get_team_num): Move
	to ...
	* config/gcn/teams.c: ... here.  New file.