string: Fix string/tst-memmove-overflow to compile with GCC 7

GCC 8 relaxed what kind of expressions can be used in initializers,
and the previous use of static const variables relied on that.  Switch
to wide (non-int) enum constants instead, which is another GCC
extension that is more widely implemented.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>