syscall: drop fallback to pipe in Pipe on linux/arm

Follow-up for CL 346789

The minimum required Linux kernel version for Go 1.18 will be changed to
2.6.32, see #45964. The pipe2 syscall was added in 2.6.27, so the
fallback to use pipe in Pipe on linux/arm can be removed.

For #45964

Change-Id: I8b18244ca1f849f10e90565b4fef80ce777fef69
Reviewed-on: https://go-review.googlesource.com/c/go/+/347349


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>