aarch64: Update LS64 feature with system register

This patch:
+ Adds new ACCDATA_EL1 (Accelerator Data) system register, see [0].
+ Adds LS64 instruction tests.
+ Update LS64 feature test with new register.
+ Fix comment for AARCH64_OPND_Rt_LS64.

    [0] https://developer.arm.com/docs/ddi0595/i/aarch64-system-registers/accdata_el1

Note: as this is register only extension we do not want to hide these
registers behind -march flag going forward (they should be enabled by
default).