Record right reg num of thumb special data instructions

When GDB decodes these thumb special data instructions, such as 'mov sp, r7'
the Rd is got incorrectly.  According to the arch reference manual, the Rd
is DN:Rdn, in which DN is bit 7 and Rdn is bits 0 to 2.  This patch fixes it.

gdb:

2016-02-26  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (thumb_record_ld_st_reg_offset): Fix the register
	number of Rd.