Macro RISCV_HWPROBE_EXT_ZVFHMIN is supposed to be use when making a hwprobe syscall
for detecting the Zvfhmin extension. Current definition yields a negative 32bits value,
messing up hwprobe result when Zvfhmin extension presents. Replace it by using a 1ULL
bit shift value as done in kernel upstream [1]. Although it is not used by the JVM for
now, it's better to correct it for consistency with the kernel upstream.