unix: implement Pipe using pipe2 syscall on all linux platforms

Most architectures currently already implement Pipe using the pipe2
syscall. Only 386, amd64, mips{,le}, ppc and sparc64 still use the pipe
syscall. However, some systems (e.g. Android seccomp policies) block
that syscall, see golang/go#40828 for an example.

The pipe2 syscall was added in Linux kernel version 2.6.27. The minimum
required Linux kernel version for Go 1.18 will be changed to 2.6.32
per golang/go#45964 so it is possible to unify the implementation of
Pipe using the pipe2 syscall.

Same as CL 350530 did for package syscall.

For golang/go#45964
Fixes golang/go#40828

Change-Id: I07d84d3ea06a54e37d9ff6b72702fe5d96efef7b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/350549
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux.go

@@ -120,6 +120,23 @@ func Openat2(dirfd int, path string, how *OpenHow) (fd int, err error) {
 	return openat2(dirfd, path, how, SizeofOpenHow)
 }
 
+func Pipe(p []int) error {
+	return Pipe2(p, 0)
+}
+
+//sysnb	pipe2(p *[2]_C_int, flags int) (err error)
+
+func Pipe2(p []int, flags int) error {
+	if len(p) != 2 {
+		return EINVAL
+	}
+	var pp [2]_C_int
+	err := pipe2(&pp, flags)
+	p[0] = int(pp[0])
+	p[1] = int(pp[1])
+	return err
+}
+
 //sys	ppoll(fds *PollFd, nfds int, timeout *Timespec, sigmask *Sigset_t) (n int, err error)
 
 func Ppoll(fds []PollFd, timeout *Timespec, sigmask *Sigset_t) (n int, err error) {