* linux-low.h (struct linux_target_ops): Replace left_pad_xfer field
by collect_ptrace_register and supply_ptrace_register hooks.
* linux-low.c (fetch_register): Use supply_ptrace_register callback
instead of checking for the_low_target.left_pad_xfer.
(usr_store_inferior_registers): Use collect_ptrace_register callback
instead of checking for the_low_target.left_pad_xfer.