unix: defer Getpagesize() to runtime

In general, page size is not a function of the archetecture. This was
addressed in the Go standard library here:
https://go-review.googlesource.com/25051

This change simply defers to the standard library "syscall" package,
which in turn defers to the runtime. This helps in addressing golang/go#10180 and
also fixes a bug on ppc64.

Currently, we return 65536 as the page size on ppc64, but the kernel
supports 4k and 64k sizes, see here:
http://elixir.free-electrons.com/linux/v4.13/source/arch/powerpc/include/asm/page.h#L24

Now that various page size calculations are not needed, various
components are now dead code and can also be removed. This CL reverts:
https://go-review.googlesource.com/14483
and part of:
https://go-review.googlesource.com/30755

Change-Id: I9d7a2d96359054e0dca9c985b026c8072b2eeaf3
Reviewed-on: https://go-review.googlesource.com/62111
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_darwin_amd64.go

@@ -11,8 +11,6 @@ import (
 	"unsafe"
 )
 
-func Getpagesize() int { return 4096 }
-
 func TimespecToNsec(ts Timespec) int64 { return int64(ts.Sec)*1e9 + int64(ts.Nsec) }
 
 func NsecToTimespec(nsec int64) (ts Timespec) {