metag: Fix METAG Kconfig symbol select ordering

Commit d1a1dc0be866 (consolidate per-arch stack overflow debugging
options) broke the ordering of the selects in arch/metag/Kconfig by
adding select HAVE_DEBUG_STACKOVERFLOW at the end. Move it to the right
place.

Signed-off-by: James Hogan <james.hogan@imgtec.com>