Share windows_thread_info between gdb and gdbserver

Share windows_thread_info between gdb and gdbserver

This introduces a new file, nat/windows-nat.h, which holds the
definition of windows_thread_info.  This is now shared between gdb and
gdbserver.

Note that the two implementations different slightly.  gdb had a
couple of fields ("name" and "reload_context") that gdbserver did not;
while gdbserver had one field ("base_context") that gdb did not, plus
better comments.  The new file preserves all the fields, and the
comments.

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

	* windows-nat.c (struct windows_thread_info): Remove.
	* nat/windows-nat.h: New file.

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

	* win32-low.h (struct windows_thread_info): Remove.