sh: Restore $((x)) error checking after fix for $((-9223372036854775808))
SVN r342880 was designed to fix $((-9223372036854775808)) and things like
$((0x8000000000000000)) but also broke error detection for values of
variables without dollar sign ($((x))).
For compatibility, overflow in plain literals continues to be ignored and
the value is clamped to the boundary (except 9223372036854775808 which is
changed to -9223372036854775808).
Reviewed by: se (although he would like error checking to be removed)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D18926