syscall: split implementation of Pipe/Pipe2 per GOOS

Updates #9974

This proposal moves the definition of Pipe an Pipe2 from the generic
syscall_linux.go to the GOOS specific variants. This is in preparation
for the arm64 port.

For platforms where pipe2(2) is not supported in the minimum 2.6.23 kernel,
amd64 and 386, we retain pipe(2). For all other platforms pipe(2) is removed
and Pipe forwards to pipe2(2).

Because mksycall.pl does not sort symbols before generating the output file
the diff includes some unavoidable code moves as Pipe and Pipe2 are processed
latter in the run.

Discussion: https://groups.google.com/forum/#!topic/golang-dev/zpeFtN2z5Fc

Change-Id: Ie26d6761eeb9760dbaff974ee8bc0d57a9ceaee4
Reviewed-on: https://go-review.googlesource.com/5833


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>