unix: fix Getpagesize to return actual system value on Solaris

In preparation for issues such as #10180, Getpagesize() needs to be
fixed to return the actual system's page size instead of assuming it's
always 4096.

This is particularly important for future platform support on Solaris.

Fixes #12076

Change-Id: I78205165909529215fe93ed6ba56e9c3ee1c2abb
Reviewed-on: https://go-review.googlesource.com/14483
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_solaris.go

@@ -13,6 +13,7 @@
 package unix
 
 import (
+	"sync/atomic"
 	"syscall"
 	"unsafe"
 )
@@ -548,3 +549,18 @@ 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)
+}