unix: fix last argument of pselect6 on linux

On Linux, the last argument of pselect6 system call is **not** a
sigseg_t * pointer, but instead it is a structure of the form:

    struct {
        const sigset_t *ss;     /* Pointer to signal set */
        size_t          ss_len; /* Size (in bytes) of object pointed
    };

See man 2 pselect6.

Fixes #61251

Change-Id: Id0aa122a77796713bc6d624dc395d396fbc0c5e2
GitHub-Last-Rev: cb3c6d7da9b846843a4a81898ebfdcf2e449942a
GitHub-Pull-Request: golang/sys#167
Reviewed-on: https://go-review.googlesource.com/c/sys/+/510195
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

unix/linux/types.go

@@ -968,6 +968,10 @@ const (
 )
 
 type Sigset_t C.sigset_t
+type sigset_argpack struct {
+	ss    *Sigset_t
+	ssLen uintptr // Size (in bytes) of object pointed to by ss.
+}
 
 const _C__NSIG = C._NSIG