This is supposed to be the equivalent of __NR_syscalls, not
__NR_syscalls -1. The x86 code this was based on had simply fallen
out of sync at the time this was implemented. Fix it up now.

As a result, tracing of __NR_perf_counter_open works as advertised.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>