[dev.cc] runtime: fix bus error accessing auxv random data on arm5

It's rather unsporting of the kernel to give us a pointer to unaligned memory.

This fixes one crash, the next crash occurs in the soft float emulation.

LGTM=minux, rsc, austin
R=minux, rsc, austin
CC=golang-codereviews
https://golang.org/cl/177730043

src/runtime/os_linux_arm.go

@@ -50,9 +50,12 @@ func setup_auxv(argc int32, argv **byte) {
 
 	for i := 0; auxv[i] != _AT_NULL; i += 2 {
 		switch auxv[i] {
-		case _AT_RANDOM: // kernel provided 16-byte worth of random data
+		case _AT_RANDOM: // kernel provides a pointer to 16-bytes worth of random data
 			if auxv[i+1] != 0 {
-				randomNumber = *(*uint32)(unsafe.Pointer(uintptr(auxv[i+1])))
+				// the pointer provided may not be word alined, so we must to treat it
+				// as a byte array.
+				rnd := (*[16]byte)(unsafe.Pointer(uintptr(auxv[i+1])))
+				randomNumber = uint32(rnd[0]) | uint32(rnd[1])<<8 | uint32(rnd[2])<<16 | uint32(rnd[3])<<24
 			}
 
 		case _AT_PLATFORM: // v5l, v6l, v7l