java/openjdk11: fix build on powerpc64*

Go back to using LLVM 12, bin/java built with LLVM 13 crashes with:
* For target jdk__packages_attribute.done:

 A fatal error has been detected by the Java Runtime Environment:

  SIGILL (0x4) at pc=0x0000000802e3d438, pid=28756, tid=149399

 JRE version:  (11.0.16+8) (build )
 Java VM: OpenJDK 64-Bit Server VM (11.0.16+8-1, mixed mode, tiered, compressed oops, serial gc, bsd-ppc64)
 Problematic frame:
 V  [libjvm.so+0xe3d438]  JVM_RaiseSignal+0x446bcc