The selftest.m4 file is used by gdb, gdbserver and gdbsupport, I think
it belongs in gdbsupport.
gdb/ChangeLog:
* selftest.m4: Move to gdbsupport/.
* acinclude.m4: Update path to selftest.m4.
gdbserver/ChangeLog:
* acinclude.m4: Update path to selftest.m4.
gdbsupport/ChangeLog:
* selftest.m4: Moved from gdb/.
* acinclude.m4: Update path to selftest.m4.