Remove global variable arm_linux_has_wmmx_registers in arm-linux-nat.c

Remove global variable arm_linux_has_wmmx_registers in arm-linux-nat.c

This patch is to remove the global variable arm_linux_has_wmmx_registers
in arm-linux-nat.c, and add a new field have_wmmx_registers in
'struct gdbarch_tdep'.

gdb:

2015-05-28  Yao Qi  <yao.qi@linaro.org>

	* arm-linux-nat.c (arm_linux_has_wmmx_registers): Remove.
	(arm_linux_fetch_inferior_registers): Use
	tdep->have_wmmx_registers instead of arm_linux_has_wmmx_registers.
	(arm_linux_store_inferior_registers): Likewise.
	(arm_linux_read_description): Don't set
	arm_linux_has_wmmx_registers.
	* arm-tdep.c (arm_gdbarch_init): Set
	tdep->have_wmmx_registers according target descriptions.
	* arm-tdep.h (struct gdbarch_tdep) <have_wmmx_registers>: New
	field.