Add missing comments in rs6000-tdep.c, ppc64-tdep.c and ppc-linux-tdep.c.

Add missing comments in rs6000-tdep.c, ppc64-tdep.c and ppc-linux-tdep.c.

gdb/ChangeLog:

	* ppc-linux-tdep.c (ppc_skip_trampoline_code,
	ppc_canonicalize_syscall, ppc_linux_syscall_record,
	ppc_linux_record_signal, ppc_init_linux_record_tdep): Add comments.
	* ppc64-tdep.c (ppc64_skip_trampoline_code): Likewise.
	* rs6000-tdep.c (rs6000_epilogue_frame_cache,
	rs6000_epilogue_frame_this_id, rs6000_epilogue_frame_prev_register,
	rs6000_epilogue_frame_sniffer, ppc_record_vsr, ppc_process_record_op4,
	ppc_process_record_op19, ppc_process_record_op31,
	ppc_process_record_op59, ppc_process_record_op60,
	ppc_process_record_op63): Likewise.