Improve support for loading DLLs at run time in gdbserver.

This fixes win32-low.cc in the same way as a recent change in
windows-nat.c did for GDB: if the lpImageName member of the load-DLL
debug event doesn't allow us to find the file name of the DLL, then
loop over all the DLLs mapped into the inferior to find the one loaded
at the same base address as given by the lpBaseOfDll member of the
debug event.

gdbserver/ChangeLog:

2021-04-11  Eli Zaretskii  <eliz@gnu.org>

	* win32-low.cc (win32_add_dll): New function, with body almost
	identical to what win32_add_all_dlls did.  Accepts one argument;
	if that is non-NULL, returns the file name of the DLL that is
	loaded at the base address equal to that argument, or NULL if not
	found.  If the argument is NULL, add all the DLLs loaded by the
	inferior to the list of solibs and return NULL.
	(win32_add_all_dlls): Now a thin wrapper around win32_add_dll.
	(windows_nat::handle_load_dll) [!_WIN32_WCE]: If get_image_name
	failed to glean the file name of the DLL, call win32_add_dll to
	try harder using the lpBaseOfDll member of the load-DLL event.