unix: add SIG_BLOCK and friends for Linux

The recently added PthreadSigmask syscall expects a parameter
how that specifies which action to perform on the thread's signal
set. The value of this parameter differs between arches. Add the
SIG_* constants to have a portable source.

Updates golang/go#55349

Change-Id: I7cb42d7c2c42e9b3d22123e8de74960121d89313
Reviewed-on: https://go-review.googlesource.com/c/sys/+/435775
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: lmb

unix/linux/types.go

@@ -959,6 +959,12 @@ type Sigset_t C.sigset_t
 
 const _C__NSIG = C._NSIG
 
+const (
+	SIG_BLOCK   = C.SIG_BLOCK
+	SIG_UNBLOCK = C.SIG_UNBLOCK
+	SIG_SETMASK = C.SIG_SETMASK
+)
+
 type SignalfdSiginfo C.struct_signalfd_siginfo
 
 type Siginfo C.siginfo_t