RFR: [8u] Workaround VM crash with JNI Weak Refs handling

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/8u-workaround-jweak/webrev.01/

Found this during sh/jdk8u backports, which brought the TestJNIGlobalRefs test. This test crashes
current VM with SEGV, or fails in Verifier like this:

#  Internal Error
(/home/shade/trunks/shenandoah-jdk8/hotspot/src/share/vm/gc_implementation/shenandoah/shenandoahVerifier.cpp:146),
pid=34021, tid=0x00007f2a54bfb700
#  Error: Shenandoah verification failed; After Mark, Roots: Object must belong to region with live data

Referenced from:
  interior location: 0x00007f2a6c6d3810
  outside of Java heap
  0x00007f2a6c6d3810 is pointing into the stack for thread: 0x00007f2a6400c800

Object:
  0x0000000082400008 - klass 0x0000000800000b70 [I
    not allocated after complete mark start
        allocated after next mark start
    not marked complete
        marked next
    not in collection set
  region: |0x00007f2a64086060|    1|R  |BTE 0x0000000082400000, 0x0000000082500000,
0x0000000082800000|U  25%|T  25%|G   0%|S   0%|L   0%|CP   0|TAMS 0x0000000082500000,
0x0000000082400000|

It seems to be caused by a race between reading the jweak into the local (and thus it appears on
thread stack), and marking that is supposed to reveal its strong reachability from that root.

My attempts in fixing this easily have not been successful so far. There are upstream jdk9/10
changes that rework jweak handling and fix some related G1 bugs, but those are not trivially
backportable. Until we figure that out, I suggest we workaround that failure by assuming JNI Weak
Refs are always alive.

Testing: hotspot_gc_shenandoah, including new test

Thanks,
-Aleksey