looping in alpha_vms_slurp_relocs

The direct cause for the looping was failing to test for error return
from _bfd_vms_get_object_record inside a while(1) loop.  Fix that.
Also record status of first alpha_vms_slurp_relocs call and return
that for all subsequent calls.  (The object format has one set of
relocation records for all sections.)  If the first call fails, all
others should too.

	* vms-alpha.c (struct vms_private_data_struct): Make reloc_done
	a tri-state int.
	(alpha_vms_slurp_relocs): Set reloc_done to 1 on success, -1 on
	failure.  Return that status on subsequent calls.  Check
	_bfd_vms_get_object_record return status.
	(alpha_vms_get_reloc_upper_bound): Return status from
	alpha_vms_slurp_relocs.
	(alpha_vms_write_exec): Exclude sections with contents NULL due
	to previous errors from layout, and don't try to write them.