runtime/cgo: fix deadlock involving signals on darwin
sigprocmask() is process-wide on darwin, so two concurrent
libcgo_sys_thread_start() can result in all signals permanently
blocked, which in particular blocks handling of nil derefs.
Fixes issue 4833.

R=golang-dev, dave, rsc
CC=golang-dev
https://codereview.appspot.com/7324058

Committer: Russ Cox <rsc@golang.org>
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 <rminnich@gmail.com>