risugen: control re-randomization of registers with a flag

use a global flag to control whether to re-randomize
registers periodically. This is useful for debugging purposes.

Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>
aarch64.risu: add ST1,ST2,ST3,ST4 and SIMD/fp LDNP/STNP

Add the Store Multiple/Single Structure SIMD/fp insns,
as well as the SIMD/fp variants of the LDNP/STNP insns.
LD1, LD2, LD3, LD4 still TODO.

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