The arm_cp15 function for accessing the current CPU index is specific
to ARMv7 while this header is used for ARMv8 as well. Instead, use a
generic accessor that is part of the standard CPU API.
Use the targets parameter to determine the targets of the SGI. Change
targets parameter type to 32-bit to ease the parameter passing. GICv3
supports up to 16 targets.
Update #4202.
