Clear GDB internal state after each unit test

Clear GDB internal state after each unit test

GDB has some global variables, like sentinel_frame,
current_thread_arch, and etc, we need to reset them after each unit
tests.

gdb:

2017-05-24  Yao Qi  <yao.qi@linaro.org>

	* selftest-arch.c (tests_with_arch): Call registers_changed
	and reinit_frame_cache.
	* selftest.c (run_self_tests): Likewise.