allow full-range file offsets to mmap on archs with 64-bit syscall args

allow full-range file offsets to mmap on archs with 64-bit syscall args

normally 32-bit archs use the mmap2 syscall and are limited to an
offset of 2^32 pages. however some 32-bit archs (mainly ILP32-on-64
ones like x32) have 64-bit syscall argument slots and thus can accept
the full range. don't artifically limit them.