linux: pread/write: fix 64bit handling

The syscall on 64bit ports takes 4 args as there is no need to split
up the value into two args.  Add support for that to the common code.

Once we fix that, the mips code can now leverage it for its 64bit and
32bit needs.  However, we can't just drop it entirely yet because its
n32 ABI needs special handling to treat it like a 64bit port.  This
does change the existing behavior which treats the n32 like a 32bit
port, but we want to do this.

In the future, we'll probably have to introduce a define for this as
it currently affects x86_64/x32 and mips/n32.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>