Disassembly unit test: disassemble one instruction

This patch adds one unit test, which disassemble one instruction for
every gdbarch if available.  The test needs one valid instruction of
each gdbarch, and most of them are got from breakpoint instruction.
For the rest gdbarch whose breakpoint instruction isn't a valid
instruction, I copy one instruction from the gas/testsuite/gas/
directory.

I get the valid instruction of most gdbarch except ia64, mep, mips,
tic6x, and xtensa.  People familiar with these arch should be easy
to extend the test.

In order to achieve "do the unit test for every gdbarch", I add
selftest-arch.[c,h], so that we can register a function pointer,
which has one argument gdbarch.  selftest.c will iterate over all
gdbarches to call the registered function pointer.

gdb:

2017-01-26  Yao Qi  <yao.qi@linaro.org>

	* Makefile.in (SFILES): Add disasm-selftests.c and
	selftest-arch.c.
	(COMMON_OBS): Add disasm-selftests.o and selftest-arch.o.
	* disasm-selftests.c: New file.
	* selftest-arch.c: New file.
	* selftest-arch.h: New file.