Move initialize_tdesc_mips* calls from mips-linux-nat.c to mips-linux-tdep.c

Move initialize_tdesc_mips* calls from mips-linux-nat.c to mips-linux-tdep.c

All target descriptions except mips initialization are called in -tdep.c,
instead of -nat.c, so this patch moves mips target descriptions
initialization to -tdep.c.  Secondly, I want to change the target
descriptions from pre-generated to dynamical creation, so I want to test
that these pre-generated target descriptions equal to these dynamically
created ones.  Move target descriptions initialization to -tdep.c files so
we can test them in any hosts (if they are still -nat.c, we can only test
them on mips-linux host.).

gdb:

2017-06-13  Yao Qi  <yao.qi@linaro.org>

	* mips-linux-nat.c: Move include features/mips*-linux.c to
	mips-linux-tdep.c.
	(_initialize_mips_linux_nat): Move initialize_tdesc_mips* calls
	to mips-linux-tdep.c.
	* mips-linux-tdep.c: Include features/mips*-linux.c
	(_initialize_mips_linux_tdep): Call initialize_tdesc_mips*
	functions.
	* mips-linux-tdep.h (tdesc_mips_linux): Declare.
	(tdesc_mips_dsp_linux, tdesc_mips64_linux): Declare.
	(tdesc_mips64_dsp_linux): Declare.