[binutils-gdb] Allow "set sysroot" with no argument to restore sysroot to empty

[sergiodj+buildbot@redhat.com]

*** TEST RESULTS FOR COMMIT 811a659a779fdf93293fe1105d99e9db171a8b68 ***

Author: Gary Benson <gbenson@redhat.com>
Branch: master
Commit: 811a659a779fdf93293fe1105d99e9db171a8b68

Allow "set sysroot" with no argument to restore sysroot to empty
This commit creates the "set/show sysroot" commands using
add_setshow_optional_filename_cmd to allow the sysroot to
be restored to empty after being set.

gdb/ChangeLog:

	* solib.c (_initialize_solib): Make "set/show sysroot" use
	add_setshow_optional_filename_cmd so it can be restored to
	empty after being set.