Add xml selftests for aarch64 target description.

gdb/
	* aarch64-tdep.c (_initialize_aarch64_tdep): Add target desc
	selftest.

gdbserver/
	* configure.srv: Add linux-aarch64-tdesc-selftest.o.
	* linux-aarch64-low.c (initialize_low_arch): Call init func.
	* linux-aarch64-tdesc-selftest.c: New file.
	* linux-aarch64-tdesc.h (initialize_low_tdesc): New declaration.