Define os.PageSize and syscall.PageSize.

R=rsc
APPROVED=rsc
DELTA=13  (13 added, 0 deleted, 0 changed)
OCL=29429
CL=29819
diff --git a/src/lib/syscall/syscall_linux_amd64.go b/src/lib/syscall/syscall_linux_amd64.go
index 8b3404d..2442640 100644
--- a/src/lib/syscall/syscall_linux_amd64.go
+++ b/src/lib/syscall/syscall_linux_amd64.go
@@ -6,6 +6,10 @@
 
 import "syscall"
 
+func Getpagesize() int {
+	return 4096
+}
+
 func TimespecToNsec(ts Timespec) int64 {
 	return int64(ts.Sec)*1e9 + int64(ts.Nsec);
 }