libgomp, OpenMP: Fix issue for omp_get_device_num on gcn targets.

libgomp, OpenMP: Fix issue for omp_get_device_num on gcn targets.

Currently omp_get_device_num does not work on gcn targets with more than one
offload device. The reason is that GOMP_DEVICE_NUM_VAR is static in
icv-device.c and thus "__gomp_device_num" is not visible in the offload image.

This patch removes "static" such that "__gomp_device_num" is now part of the
offload image and can now be found in GOMP_OFFLOAD_load_image in the plugin.

This is not an issue for nvptx. There, "__gomp_device_num" is in the offload
image even with "static".

libgomp/ChangeLog:

	* config/gcn/icv-device.c: Make GOMP_DEVICE_NUM_VAR public (remove
	"static") to make the device num available in the offload image.