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

Same as CL 347349 does for package syscall.

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

For golang/go#45964

Change-Id: I8e222dc4b373b22f91bc1ddcc48f58a37dafcffa
Reviewed-on: https://go-review.googlesource.com/c/sys/+/347331
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>

golang/sys,golang/sys,golang/sys