Fix aarch64 to properly access arguments for wrapped syscalls

Fix aarch64 to properly access arguments for wrapped syscalls

Linux 4.18 added wrappers for aarch64 syscalls that pass a pointer to
a struct pt_regs holding the values for the actual arguments.  The
syscall tapsets initialize CONTEXT->sregs to point at this data
structure.  However, the aarch64 specific register access code was
using the CONTEXT->kregs and just getting the processor register state
when the kprobe triggered rather than the expected arguments in the
data structure being passed into the syscall.  The aarch64 specific
register code now gets the syscall arguments from the correct pt_regs
structure.