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/zsyscall_solaris_amd64.go

@@ -129,7 +129,6 @@ import (
 //go:cgo_import_dynamic libc_getpeername getpeername "libsocket.so"
 //go:cgo_import_dynamic libc_setsockopt setsockopt "libsocket.so"
 //go:cgo_import_dynamic libc_recvfrom recvfrom "libsocket.so"
-//go:cgo_import_dynamic libc_sysconf sysconf "libc.so"
 
 //go:linkname procpipe libc_pipe
 //go:linkname procgetsockname libc_getsockname
@@ -250,7 +249,6 @@ import (
 //go:linkname procgetpeername libc_getpeername
 //go:linkname procsetsockopt libc_setsockopt
 //go:linkname procrecvfrom libc_recvfrom
-//go:linkname procsysconf libc_sysconf
 
 var (
 	procpipe,
@@ -371,8 +369,7 @@ var (
 	proc__xnet_getsockopt,
 	procgetpeername,
 	procsetsockopt,
-	procrecvfrom,
-	procsysconf syscallFunc
+	procrecvfrom syscallFunc
 )
 
 func pipe(p *[2]_C_int) (n int, err error) {
@@ -1589,12 +1586,3 @@ func recvfrom(fd int, p []byte, flags int, from *RawSockaddrAny, fromlen *_Sockl
 	}
 	return
 }
-
-func sysconf(name int) (n int64, err error) {
-	r0, _, e1 := sysvicall6(uintptr(unsafe.Pointer(&procsysconf)), 1, uintptr(name), 0, 0, 0, 0, 0)
-	n = int64(r0)
-	if e1 != 0 {
-		err = e1
-	}
-	return
-}