runtime: get randomness from AT_RANDOM AUXV on linux/mips64x

Fixes #15148.

Change-Id: If3b628f30521adeec1625689dbc98aaf4a9ec858
Reviewed-on: https://go-review.googlesource.com/22811Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/os_linux_mips64x.go

@@ -9,6 +9,17 @@ package runtime
 
 var randomNumber uint32
 
+func archauxv(tag, val uintptr) {
+	switch tag {
+	case _AT_RANDOM:
+		// sysargs filled in startupRandomData, but that
+		// pointer may not be word aligned, so we must treat
+		// it as a byte array.
+		randomNumber = uint32(startupRandomData[4]) | uint32(startupRandomData[5])<<8 |
+			uint32(startupRandomData[6])<<16 | uint32(startupRandomData[7])<<24
+	}
+}
+
 //go:nosplit
 func cputicks() int64 {
 	// Currently cputicks() is used in blocking profiler and to seed fastrand1().

src/runtime/os_linux_noauxv.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !amd64,!arm,!arm64
+// +build !amd64,!arm,!arm64,!mips64,!mips64le
 
 package runtime