parisc: tracehook_report_syscall

This makes parisc use the standard tracehook_report_syscall_entry
and tracehook_report_syscall_exit hooks in <linux/tracehook.h>.

To do this, we need to access current->thread.regs, and to know
whether we're entering or exiting the syscall, so add this to
syscall_trace.

Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>