Tweak module initialization to comply with JVM spec

Dmitry learned that we've been relying on a bug in the
verifier that will be fixed in JDK 9 under the new
classfile format.

Assignment to a static final must occur lexically
within the <clinit>. We were performing this assignment
from the constructor of the module class.

This commit moves the assignment to <clinit>.

Before:

```
public final class O$ {
  public static final O$ MODULE$;

  public static {};
    Code:
       0: new           #2                  // class O$
       3: invokespecial #12                 // Method "<init>":()V
       6: return

  private O$();
    Code:
       0: aload_0
       1: invokespecial #13                 // Method java/lang/Object."<init>":()V
       4: aload_0
       5: putstatic     #15                 // Field MODULE$:LO$;
       8: return
}
```

After:

```
public final class O$ {
  public static final O$ MODULE$;

  public static {};
    Code:
       0: new           #2                  // class O$
       3: dup
       4: invokespecial #12                 // Method "<init>":()V
       7: putstatic     #14                 // Field MODULE$:LO$;
      10: return

  private O$();
    Code:
       0: aload_0
       1: invokespecial #15                 // Method java/lang/Object."<init>":()V
       4: return
}
```

The difference is observable is the constructor is called a second
time with reflection/setAccessible. That used to overwrite the
`MODULE$` field. I think few would argue that the new semantics
are a clear improvement.

Author: retronym

src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala

@@ -563,13 +563,6 @@ abstract class BCodeBodyBuilder extends BCodeSkelBuilder {
               fun.symbol.javaSimpleName.toString == INSTANCE_CONSTRUCTOR_NAME &&
               isStaticModuleClass(claszSymbol)) {
               isModuleInitialized = true
-              mnode.visitVarInsn(asm.Opcodes.ALOAD, 0)
-              mnode.visitFieldInsn(
-                asm.Opcodes.PUTSTATIC,
-                thisBType.internalName,
-                strMODULE_INSTANCE_FIELD,
-                thisBType.descriptor
-              )
             }
           }
           // 'super' call: Note: since constructors are supposed to

src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala

@@ -217,8 +217,10 @@ abstract class BCodeSkelBuilder extends BCodeHelpers {
       /* "legacy static initialization" */
       if (isCZStaticModule) {
         clinit.visitTypeInsn(asm.Opcodes.NEW, thisBType.internalName)
+        clinit.visitInsn(asm.Opcodes.DUP)
         clinit.visitMethodInsn(asm.Opcodes.INVOKESPECIAL,
                                thisBType.internalName, INSTANCE_CONSTRUCTOR_NAME, "()V", false)
+        clinit.visitFieldInsn(asm.Opcodes.PUTSTATIC, thisBType.internalName, strMODULE_INSTANCE_FIELD, thisBType.descriptor)
       }
       if (isCZParcelable) { legacyAddCreatorCode(clinit, cnode, thisBType.internalName) }
       clinit.visitInsn(asm.Opcodes.RETURN)