risugen: add fp/SIMD init and periodic update

initialize fpsr to 0 and fpcr to the user-supplied parameter
(default 0).
initialize and periodically update the V[] registers with
random floating point values.
Note that in contrast to arm we do not (yet) eskew the
random data for interesting values (NaN, 0, infinities, ..)

Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>