Handle %I64d in format_pieces

Handle %I64d in format_pieces

We found a bug internally where gdb would crash while disassembling a
certain instruction.  This was tracked down to the handling of %I64d
in format_pieces.

format_pieces will convert %ll to %I64d on mingw -- so format_pieces
should also handle parsing this format.  In this patch, I've made the
parsing unconditional, since I think it is harmless to accept extra
formats.  I've also taken the opportunity to convert the length
modifier test to a "switch".

Tested internally using our failing test case.

gdb/ChangeLog
2019-11-21  Tom Tromey  <tromey@adacore.com>

	* gdbsupport/format.c (format_pieces): Parse %I64d.
	* unittests/format_pieces-selftests.c (test_windows_formats): New
	function.
	(run_tests): Call it.

Change-Id: If335c7c2fc8d01e629cd55182394a483334d79c7