interp: support big-endian targets

The interp package was assuming that all targets were little-endian. But
that's not true: we now have a big-endian target (GOARCH=mips).

This fixes the interp package to use the appropriate byte order for a
given target.

compiler/llvmutil/llvm.go

@@ -8,6 +8,7 @@
 package llvmutil
 
 import (
+	"encoding/binary"
 	"strconv"
 	"strings"
 
@@ -216,3 +217,13 @@ func Version() int {
 	}
 	return major
 }
+
+// Return the byte order for the given target triple. Most targets are little
+// endian, but for example MIPS can be big-endian.
+func ByteOrder(target string) binary.ByteOrder {
+	if strings.HasPrefix(target, "mips-") {
+		return binary.BigEndian
+	} else {
+		return binary.LittleEndian
+	}
+}