arch: implement brute force find_lsb_set()

On RISC-V 64-bit, GCC complains about undefined reference
to 'ffs' via __builtin_ffs(). So implement a brute force
way to do it. Once the toolchain has __builtin_ffs(),
this can be reverted.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>