t/op/chop.t: Don't use too large code points

t/op/chop.t: Don't use too large code points

The bug this was testing for requires a code point that will no longer
be legal on 32-bit machines.  So skip unless we're on a 64-bit platform,
and revise to use chr() in the skipped code instead of "\x{}".  The
latter would try to be compiled even if execution gets skipped, so would
cause it to die, whereas chr() is runtime, so get skipped if
inappropriate.  This also tested the very highest legal code point on
64-bit machines, which is now illegal, so test the new very highest one.