New proc is_aarch64_target

New proc is_aarch64_target

Some tests expect the the target is aarch64, but checking target
triplet is not accurate, because target triplet can be aarch64 but
the program is in arm (or aarch32) state.

This patch addes a new proc is_aarch64_target which returns true
if the target is on aarch64 state.

gdb/testsuite:

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

	* gdb.arch/aarch64-atomic-inst.exp: Check is_aarch64_target
	instead of istarget "aarch64*-*-*".
	* gdb.arch/aarch64-fp.exp: Likewise.
	* gdb.base/float.exp: Likewise.
	* gdb.reverse/aarch64.exp: Likewise.
	* lib/gdb.exp (is_aarch64_target): New proc.