Building with --jdk-variant=zero breaks on Macos with the following warning/errors:

<jdk_src/src/hotspot/share/interpreter/zero/bytecodeInterpreter.cpp>:2930:5: error: unused label 'check_for_exception' [-Werror,-Wunused-label]
    check_for_exception:
    ^~~~~~~~~~~~~~~~~~~~
</jdk_src/src/hotspot/share/interpreter/zero/bytecodeInterpreter.cpp>:2940:3: error: unused label 'do_continue' [-Werror,-Wunused-label]
  do_continue: ;
  ^~~~~~~~~~~~~
2 errors generated.
gmake[3]: *** [lib/CompileJvm.gmk:170: </build-release/hotspot/variant-zero/libjvm/objs/bytecodeInterpreter.o]> Error 1
gmake[3]: *** Waiting for unfinished jobs....
gmake[2]: *** [make/Main.gmk:245: hotspot-zero-libs] Error 2
gmake[2]: *** Waiting for unfinished jobs....

"do_continue:" is only necessary when PREFETCH_OPCCODE is defined, so should be conditional.

"check_for_exception:" isn't used by anything and can be removed entirely.