tests: head-c: avoid spurious failure with a 32-bit size_t

tests: head-c: avoid spurious failure with a 32-bit size_t

* tests/misc/head-c.sh: Don't try to elide 1 exabytes, since on
32-bit systems, that number is not representable as a size_t.
This command would fail on 32-bit systems, where SIZE_MAX < 1E:
  head --bytes=-E < /dev/null
Instead of "E", use $SSIZE_MAX.
For discussion, see http://bugs.gnu.org/13530