JDK-8305895 started to wrap `oopDesc::set_klass_gap()` calls with `oopDesc::has_klass_gap()`, but missed a spot in Zero. So now 32-bit Zero builds fail like this:

```
=== Output from failing command(s) repeated here ===
* For target buildtools_create_symbols_javac__the.COMPILE_CREATE_SYMBOLS_batch:
#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/home/shade/trunks/jdk/src/hotspot/share/oops/oop.inline.hpp:167), pid=1592865, tid=1592897
# assert(has_klass_gap()) failed: precondition
```