syscall: Plan9, amd64: fix syscall error handling in assembly
syscall: Plan9, amd64: fix syscall error handling in assembly

Syscalls return `-1' on error and the representation is always
32-bits. The `$-1' literal in 64-bit assembly is always the
64-bit representation. So this change makes sure that we
always do a 32-bit comparison when checking for error.
Also makes sure that in the error case, we return a 64-bit
`-1' from runtime.seek.

Fixes the arithmetic for handling the error-string in
runtime.Syscall6.

R=golang-dev, rminnich, rsc, ality, minux.ma
CC=golang-dev
https://codereview.appspot.com/7399052

Committer: Ron Minnich <f569d0af9a3481c4007b95c7180d6c41009f2513@gmail.com>

d0f/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,mhennings/marcohennings-go,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,mhennings/marcohennings-go,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,scirelli/scirelli-go,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,scirelli/scirelli-go,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,scirelli/scirelli-go,mhennings/marcohennings-go,d0f/go-zh,scirelli/scirelli-go,bryanxu/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,d0f/go-zh,scirelli/scirelli-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof