[aarch64] Fix one fail in gdb.xml/tdesc-regs.exp

[aarch64] Fix one fail in gdb.xml/tdesc-regs.exp

Hi,
I see the following fail in aarch64-linux-gnu testing...

(gdb) set tdesc file /XXX/gdb/testsuite/gdb.xml/single-reg.xml^M
warning: Architecture rejected target-supplied description^M
(gdb) FAIL: gdb.xml/tdesc-regs.exp: set tdesc file single-reg.xml

core-regs isn't set for aarch64 target, and looks it is an oversight
when aarch64 port was added.

gdb/testsuite:

2015-02-25  Yao Qi  <yao.qi@linaro.org>

	* gdb.xml/tdesc-regs.exp: Set core-regs to aarch64-core.xml for
	aarch64*-*-* target.