Lazily allocate 'struct regsets_info'::disabled_regsets.

Lazily allocate 'struct regsets_info'::disabled_regsets.

There's no need for every arch to pre-allocate disabled_regsets.
Chances are the array won't be used.

(I have a hunch that with some more work we could dispense with
initialize_regsets_info.)

Tested on x86_64 Fedora 17 w/ -lmcheck.

gdb/gdbserver/
2013-06-11  Pedro Alves  <palves@redhat.com>

	* linux-low.c (regset_disabled, disable_regset): New functions.
	(regsets_fetch_inferior_registers)
	(regsets_store_inferior_registers): Use them.
	(initialize_regsets_info); Don't allocate the disabled_regsets
	array here.
	* linux-low.h (struct regsets_info) <disabled_regsets>: Extend
	comment.