parisc: Fix 64-bit userspace syscall path

Currently the glibc isn't yet ported to 64-bit for hppa, so
there is no usable userspace available yet.
But it's possible to manually build a static 64-bit binary
and run that for testing. One such 64-bit test program is
available at http://ftp.parisc-linux.org/src/64bit.tar.gz


and it shows various issues with the existing 64-bit syscall
path in the kernel.
This patch fixes those issues.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org      # v4.19+