Move ARM register numbers enum to arch/arm.h

Move ARM register numbers enum to arch/arm.h

This patch moves ARM register numbers enum to arch/arm.h, so that it
can used by GDBserver too.

This patch also creates a new directory gdb/arch in which arch-specific
or target-specific files are placed.

gdb:

2015-07-30  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.h (enum gdb_regnum): Move it to ...
	* arch/arm.h: ... here.  New file.
	* Makefile.in (HFILES_NO_SRCDIR): Add arch/arm.h.