Implement stopped_by_sw_breakpoint for Windows gdbserver

Implement stopped_by_sw_breakpoint for Windows gdbserver

This changes the Windows gdbserver port to implement the
stopped_by_sw_breakpoint target method.  This is needed to support
pending stops.

This is a separate patch now, because Pedro suggested splitting it out
for simpler bisecting, in the case that it introduces a bug.

gdbserver/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* win32-low.h  (win32_process_target::stopped_by_sw_breakpoint)
	(win32_process_target::supports_stopped_by_sw_breakpoint):
	Declare.
	* win32-low.c (win32_supports_z_point_type): Always handle
	Z_PACKET_SW_BP.
	(win32_insert_point): Call insert_memory_breakpoint when needed.
	(win32_remove_point): Call remove_memory_breakpoint when needed.
	(win32_process_target::stopped_by_sw_breakpoint)
	(win32_process_target::supports_stopped_by_sw_breakpoint): New
	methods.
	(win32_target_ops): Update.
	(maybe_adjust_pc): New function.
	(win32_wait): Call maybe_adjust_pc.