Share thread_rec between gdb and gdbserver

Share thread_rec between gdb and gdbserver

This changes gdb and gdbserver to use the same calling convention for
the "thread_rec" helper function.  Fully merging these is difficult
due to differences in how threads are managed by the enclosing
applications; but sharing a declaration makes it possible for future
shared code to call this method.

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

	* windows-nat.c (enum thread_disposition_type): Move to
	nat/windows-nat.h.
	(windows_nat::thread_rec): Rename from thread_rec.  No longer
	static.
	(windows_add_thread, windows_nat_target::fetch_registers)
	(windows_nat_target::store_registers, handle_exception)
	(windows_nat_target::resume, get_windows_debug_event)
	(windows_nat_target::get_tib_address)
	(windows_nat_target::thread_name)
	(windows_nat_target::thread_alive): Update.
	* nat/windows-nat.h (enum thread_disposition_type): Move from
	windows-nat.c.
	(thread_rec): Declare.

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

	* win32-low.c (windows_nat::thread_rec): Rename from thread_rec.
	No longer static.  Change parameters.
	(child_add_thread, child_fetch_inferior_registers)
	(child_store_inferior_registers, win32_resume)
	(win32_get_tib_address): Update.