[PARISC] Fix bug when syscall nr is __NR_Linux_syscalls

[PARISC] Fix bug when syscall nr is __NR_Linux_syscalls

The bug was that we were comparing __NR_syscalls to be greater or equal
to the syscall number stored in %r20. __NR_syscalls is one greater than
the last syscall though, so we're loading one entry beyond the end of the
syscall table, and trying to jump to it.

Fix this by only checking that we're greater, alternatively, we could
have compared to (__NR_Linux_syscalls - 1)

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>