Avoid 'GTY' use for 'gcc/omp-oacc-neuter-broadcast.cc:field_map'

Avoid 'GTY' use for 'gcc/omp-oacc-neuter-broadcast.cc:field_map'

... and further simplify related things a bit.

Fix-up/clean-up for recent commit e2a58ed6dc5293602d0d168475109caa81ad0f0d
"openacc: Middle-end worker-partitioning support".

	gcc/
	* omp-oacc-neuter-broadcast.cc (field_map): Move variable into...
	(execute_omp_oacc_neuter_broadcast): ... here.
	(install_var_field, build_receiver_ref, build_sender_ref): Take
	'field_map_t *' parameter.  Adjust all users.
	(worker_single_copy, neuter_worker_single): Take a
	'record_field_map_t *' parameter.  Adjust all users.