Generalize gdb.reverse/aarch64.exp

Generalize gdb.reverse/aarch64.exp

I said we can generialize gdb.reverse/aarch64.exp for other
architectures https://sourceware.org/ml/gdb-patches/2015-05/msg00482.html
and here is the patch to change aarch64.exp so that it can be used to
test for other architectures as well.

gdb/testsuite:

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

	* gdb.reverse/aarch64.c: [__aarch64__] Include arm_neon.h.
	(testcase_ftype): New.
	(testcases): New array.
	(n_testcases): New.
	(main): Call each element in testcases.
	* gdb.reverse/aarch64.exp: Remove is_aarch64_target check.
	(read_testcase): New.
	Do the tests in a loop.