[gdb/testsuite] Remove dependence on tcl_unknown

[gdb/testsuite] Remove dependence on tcl_unknown

In gdb_init we install a local version of ::unknown, which relies on
::tcl_unknown, which is defined by dejagnu.

This proc may be moved into a namespace, or disappear altogether, as
indicated by dejagnu maintainers, so we can't rely on it.

Fix this by recreating tcl's version of unknown, and using that instead.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-06-17  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_tcl_unknown): New proc.
	(gdb_init): Use gdb_tcl_unknown for ::unknown override.  Make override
	conditional on presence of gdb_tcl_unknown.
	(gdb_finish): Make override undo conditional on presence of
	gdb_tcl_unknown.