Optimize sys_old_mmap

Optimize sys_old_mmap

* mem.c (sys_old_mmap): For Ia64 and 32-bit personality of x86-64,
copy narrow parameters from userspace by single umove, not by six
separate ones; then assign them to long u_arg[i]. For SH[64],
avoid copying of tcp->u_arg.
(sys_mmap): Add FIXME comment - SH64 and i386 seem to be handled
differently for no apparent reason.
* test/mmap_offset_decode.c: New test program, illustrates FIXME.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>