fix compiler problems discovered in openjdk-src arm bootimage build

This particular problem was introduced in the recent "compiler types" refactor.
It's unclear why it wasn't encountered before now.

Note that the full, unproguarded bootimage build is still blocked by #305
Merge pull request #306 from joshuawarner32/openjdk-src-arm-compiler-fix

fix compiler problems discovered in openjdk-src arm bootimage build