aarch64: complete FP/SIMD variants of instructions

add SIMD/FP variants of STUR/LDUR (store/load unscaled offset)
with the pre-index and post-index variants.

add the SIMD/FP pair LDP/STP (store/load signed offset)
with the pre-index and post-index variants.

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