More procfs.c simplification
    
    There are only two architectures using procfs.c (i386/SPARC), and none
    of their corresponding nat files overrides any target method.  Move
    the add_target calls to procfs.c directly.
    
    gdb/ChangeLog:
    yyyy-mm-dd  Pedro Alves  <palves@redhat.com>
    
    	* i386-sol2-nat.c (_initialize_amd64_sol2_nat): Don't call
    	procfs_target/add_target here.
    	* procfs.c (procfs_target): Make static.
    	(_initialize_procfs): Call add_target here.
    	* procfs.h (struct target_ops): Remove forward declaration.
    	(procfs_target): Remove declaration.
    	* sparc-sol2-nat.c (_initialize_sparc_sol2_nat): Delete.