java/openjdk12: fix build with clang 12

During an exp-run for llvm 12 (see bug 255570), it turned out that
java/openjdk11 does not build with clang 12.0.0:

Creating support/demos/image/jfc/J2Ddemo/J2Ddemo.jar
/usr/local/bin/bash: line 5: 49140 Abort trap		   (core dumped) /wrkdirs/usr/ports/java/openjdk12/work/openjdk-jdk12u-jdk-12.0.2-10-4/build/bsd-x86_64-server-release/support/interim-image/bin/java -XX:DumpLoadedClassList=/wrkdirs/usr/ports/java/openjdk12/work/openjdk-jdk12u-jdk-12.0.2-10-4/build/bsd-x86_64-server-release/support/link_opt/classlist.raw -Djava.lang.invoke.MethodHandle.TRACE_RESOLVE=true -cp /wrkdirs/usr/ports/java/openjdk12/work/openjdk-jdk12u-jdk-12.0.2-10-4/build/bsd-x86_64-server-release/support/classlist.jar build.tools.classlist.HelloClasslist > /dev/null 2>&1 > /wrkdirs/usr/ports/java/openjdk12/work/openjdk-jdk12u-jdk-12.0.2-10-4/build/bsd-x86_64-server-release/support/link_opt/default_jli_trace.txt
gmake[4]: *** [GenerateLinkOptData.gmk:66: /wrkdirs/usr/ports/java/openjdk12/work/openjdk-jdk12u-jdk-12.0.2-10-4/build/bsd-x86_64-server-release/support/link_opt/classlist] Error 134

This is due to missing backports of upstream commits:

commit c484d8904285652246c3af212a4211b9a8955149
Author: Thomas Stuefe <stuefe@openjdk.org>
Date:	Tue Mar 16 05:49:01 2021 +0000

    8263557: Possible NULL dereference in Arena::destruct_contents()

    Reviewed-by: kbarrett, coleenp

commit 34ae46e2bca691f989d4d84129baf545ff8a7469
Author: Andrew Haley <aph@openjdk.org>
Date:	Thu Mar 19 14:53:57 2020 +0000

    8241296: Segfault in JNIHandleBlock::oops_do()

    Reviewed-by: stefank

Approved by:	maintainer timeout (2 weeks)
PR:		255901
MFH:		2021Q2