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_solaris.go

@@ -13,7 +13,6 @@
 package unix
 
 import (
-	"sync/atomic"
 	"syscall"
 	"unsafe"
 )
@@ -699,18 +698,3 @@ func Mmap(fd int, offset int64, length int, prot int, flags int) (data []byte, e
 func Munmap(b []byte) (err error) {
 	return mapper.Munmap(b)
 }
-
-//sys	sysconf(name int) (n int64, err error)
-
-// pageSize caches the value of Getpagesize, since it can't change
-// once the system is booted.
-var pageSize int64 // accessed atomically
-
-func Getpagesize() int {
-	n := atomic.LoadInt64(&pageSize)
-	if n == 0 {
-		n, _ = sysconf(_SC_PAGESIZE)
-		atomic.StoreInt64(&pageSize, n)
-	}
-	return int(n)
-}