Fix Windows debugging regression

Fix Windows debugging regression

The updated pending stop series introduced a regression in Windows
debugging.  When stopped at a software breakpoint, we would adjust the
PC each time it was requested -- however, more than a single
adjustment is incorrect.  This patch introduces a new flag that is
used to ensure the adjustment only happens a single time.

No similar change is needed in gdbserver, because it adjusts the PC in
a different way.

I still can't run the gdb test suite on Windows, but I can run the
internal AdaCore test suite there; and this fixes the regressions
there.

gdb/ChangeLog
2020-04-24  Tom Tromey  <tromey@adacore.com>

	* nat/windows-nat.h (struct windows_thread_info)
	<pc_adjusted>: New member.
	* windows-nat.c (windows_fetch_one_register): Check
	pc_adjusted.
	(windows_nat_target::get_windows_debug_event)
	(windows_nat_target::wait): Set pc_adjusted.