gdbserver: convert the global dll list into a process_info field

gdbserver: convert the global dll list into a process_info field

The 'all_dlls' list is global.  This would cause the complete dll list
to be reported for individual processes.  Move the list into the
process_info struct.

Currently the dll list is used only by the win32-low target, which
does not support the multi-process feature.  Therefore, it practically
does not matter whether the list is global or per-process.  However,
there may be targets that are outside the binutils-gdb repo (e.g. we,
at Intel, have such a target) that have multi-process and use the dll
list.  So, it makes sense to do the right thing.

gdbserver/ChangeLog:
2021-03-22  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* inferiors.h (struct process_info) <all_dlls, dlls_changed>: New
	fields.
	* dll.h (loaded_dll)
	(unloaded_dll): Declare an overloaded version that takes a proc
	parameter.
	* dll.cc (loaded_dll)
	(unloaded_dll): Implement the overloaded versions.
	(clear_dlls): Clear all process' dll lists.
	(all_dlls, dlls_changed): Remove the global variables.
	* remote-utils.cc (prepare_resume_reply): Update to consider a dll
	list per proc.
	* server.cc (handle_qxfer_libraries): Ditto.
	(handle_v_attach): Ditto.
	(captured_main): Ditto.