guest: cc -O2 -o /root/holdopen holdopen.c
(no output, exit 0; binary -rwxr-xr-x 1 root wheel 7824)

module builds (diagnostic + fix) used the in-guest source tree:
  cd /usr/src/sys/bus/u4b/input/ums && make
  cc -O2 ... -DKLD_MODULE -c /usr/src/sys/bus/u4b/input/ums/../ums.c
  cc -Wl,--build-id=sha1 -nostdlib -Wl,--hash-style=sysv -r -Wl,-d -o ums.ko ums.o
(stock race ran against /boot/kernel/ums.ko as shipped)