Let i386_target_description return tdesc_i386_mmx

Let i386_target_description return tdesc_i386_mmx

This patch remove the usage of tdesc_i386_mmx in i386-go32-tdep.c, and use
i386_target_description to get it instead.

gdb:

2017-09-04  Yao Qi  <yao.qi@linaro.org>

	* i386-go32-tdep.c: Include x86-xstate.h.
	(i386_go32_init_abi): Call i386_target_description.
	* i386-tdep.c (i386_target_description): Return tdesc_i386_mmx
	if xcr0 is X86_XSTATE_X87_MASK.
	* i386-tdep.h (tdesc_i386): Remove the declaration.
	(tdesc_i386_mmx): Likewise.