Allow Windows UNWIND_INFO version 2.

Allow Windows UNWIND_INFO version 2.

We've observed in Windows 2012 that ntdll.dll contains some unwind
records with the version field set to 2.  This patch adjusts the
decoder to accept records flagged with this version as well.

Version 2 appears to still be largely undocumented at this stage.
However, apart from a mysterious opcode 6, everything else still
seems to remain the same. So this patch also changes the decoder
to ignore those opcodes; before this change, the debugger would
silently stop the decoding, and let the frame unwinder make do
with what it the decoder managed to decode up to that point.

It's unclear at this point what we're losing by not being able to
decode that opcode. But the information does not appear to be critical,
at least as far as call unwinding is concerned.

gdb/ChangeLog:

	(from Tristan Gingold  <gingold@adacore.com>)
	(from Joel Brobecker  <brobecker@adacore.com>)
	* amd64-windows-tdep.c (amd64_windows_frame_decode_insns):
	Accept version 2.  Ignore operations using opcode 6.