Share parts of gdb/gdbthread.h with gdbserver

GDB and gdbserver now share 'switch_to_thread' because of
fork_inferior.  To make things clear, I created a new file name
common/common-gdbthread.h, and left the implementation specific to
each part.

gdb/ChangeLog:
2017-06-07  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (HFILES_NO_SRCDIR): Add "common/common-gdbthread.h".
	* common/common-gdbthread.h: New file, with parts from
	"gdb/gdbthread.h".
	* gdbthread.h: Include "common-gdbthread.h".
	(switch_to_thread): Moved to "common/common-gdbthread.h".

gdb/gdbserver/ChangeLog:
2017-06-07  Sergio Durigan Junior  <sergiodj@redhat.com>

	* inferiors.c (switch_to_thread): New function.