Add BUILD_BROKEN_PREBUILT_ELF_FILES to dumpvars.go

This commit adds BUILD_BROKEN_PREBUILT_ELF_FILES to
`ui/build/dumpvars.go` so that its value can be printed to
`out/soong.log`.

Bug: 141176116
Test: lunch aosp_crosshatch-userdebug && make && \
      grep BUILD_BROKEN_PREBUILT_ELF_FILES out/soong.log

Change-Id: I99600a5b540be2bb93647ee8175b9f9afd1a6eb3