Once I added bootcycle-images to x86_32, this failure was discovered:
 
# Internal Error (/home/shade/trunks/shenandoah-jdk8/hotspot/src/share/vm/runtime/os.cpp:1280), pid=490815, tid=0xf5fefb40
# assert(SerializePageShiftCount == count) failed: thread size changed, fix SerializePageShiftCount constant

It happens when all three things hold true:
  - JFR is enabled at build time
  - Shenandoah is enabled at build time (i.e. the build config is not "minimal1")
  - 32-bit VM is being built