gcroot on ARM64 segfaults in DAC

[mikem8361]

Under lldb or dotnet-dump analyze "gcroot" of a object from the clrstack -a command segfaults and crashes the debugger.

* thread #1, name = 'dotnet', stop reason = signal SIGSEGV: invalid address (fault address: 0xffffffffffffffff)
  * frame #0: 0x0000ffff7b527760 libmscordaccore.so`GcInfoDecoder::ReportSlotToGC(GcSlotDecoder&, unsigned int, REGDISPLAY*, bool, unsigned int, void (*)(void*, __DPtr<Object>*, unsigned int, _DAC_SLOT_LOCATION), void*) [inlined] GcInfoDecoder::IsScratchStackSlot(spBase=<unavailable>) at gcinfodecoder.cpp:0
    frame #1: 0x0000ffff7b52772c libmscordaccore.so`GcInfoDecoder::ReportSlotToGC(this=0x0000ffffc0289528, slotDecoder=0x0000ffffc02890a0, slotIndex=8, pRD=0x0000ffffc0289a60, reportScratchSlots=false, inputFlags=<unavailable>, pCallBack=(libmscordaccore.so`DacStackReferenceWalker::GCEnumCallbackSOS(void*, __DPtr<Object>*, unsigned int, _DAC_SLOT_LOCATION) at daccess.cpp:8363), hCallBack=0x0000ffffc028a778)(void*, __DPtr<Object>*, unsigned int, _DAC_SLOT_LOCATION), void*) at gcinfodecoder.h:683
    frame #2: 0x0000ffff7b524c08 libmscordaccore.so`GcInfoDecoder::EnumerateLiveSlots(this=0x0000ffffc0289528, pRD=0x0000ffffc0289a60, reportScratchSlots=<unavailable>, inputFlags=<unavailable>, pCallBack=<unavailable>, hCallBack=<unavailable>)(void*, __DPtr<Object>*, unsigned int, _DAC_SLOT_LOCATION), void*) at gcinfodecoder.cpp:947
    frame #3: 0x0000ffff7b58fa0c libmscordaccore.so`EECodeManager::EnumGcRefs(this=<unavailable>, pRD=0x0000ffffc0289a60, pCodeInfo=0x0000ffffc02898b8, flags=0, pCallBack=(libmscordaccore.so`DacStackReferenceWalker::GCEnumCallbackSOS(void*, __DPtr<Object>*, unsigned int, _DAC_SLOT_LOCATION) at daccess.cpp:8363), hCallBack=0x0000ffffc028a778, relOffsetOverride=<unavailable>)(void*, __DPtr<Object>*, unsigned int, _DAC_SLOT_LOCATION), void*, unsigned int) at eetwain.cpp:5140
    frame #4: 0x0000ffff7b4d02f8 libmscordaccore.so`DacStackReferenceWalker::Callback(pCF=0x0000ffffc0289680, pData=0x0000ffffc028a778) at daccess.cpp:8500
    frame #5: 0x0000ffff7b5a7c30 libmscordaccore.so`Thread::StackWalkFramesEx(REGDISPLAY*, StackWalkAction (*)(CrawlFrame*, void*), void*, unsigned int, __VPtr<Frame>) [inlined] Thread::MakeStackwalkerCallback(pCallback=(libmscordaccore.so`DacStackReferenceWalker::Callback(CrawlFrame*, void*) at daccess.cpp:8461), pData=0x0000ffffc028a778)(CrawlFrame*, void*), void*) at stackwalk.cpp:880
    frame #6: 0x0000ffff7b5a7c24 libmscordaccore.so`Thread::StackWalkFramesEx(this=<unavailable>, pRD=<unavailable>, pCallback=(libmscordaccore.so`DacStackReferenceWalker::Callback(CrawlFrame*, void*) at daccess.cpp:8461), pData=0x0000ffffc028a778, flags=<unavailable>, pStartFrame=<unavailable>)(CrawlFrame*, void*), void*, unsigned int, __VPtr<Frame>) at stackwalk.cpp:960
    frame #7: 0x0000ffff7b5a8034 libmscordaccore.so`Thread::StackWalkFrames(this=0x0000ffff7b036fc0, pCallback=(libmscordaccore.so`DacStackReferenceWalker::Callback(CrawlFrame*, void*) at daccess.cpp:8461), pData=0x0000ffffc028a778, flags=99584, pStartFrame=PTR_Frame @ x19)(CrawlFrame*, void*), void*, unsigned int, __VPtr<Frame>) at stackwalk.cpp:1043
    frame #8: 0x0000ffff7b4d11f4 libmscordaccore.so`unsigned int DacStackReferenceWalker::WalkStack<unsigned int, _SOS_StackRefData>(this=0x0000000032e480d0, count=0, refs=0x0000000000000000, promote=(libmscordaccore.so`DacStackReferenceWalker::GCReportCallbackSOS(__DPtr<__DPtr<Object> >, ScanContext*, unsigned int) at daccess.cpp:8422), enumFunc=(libmscordaccore.so`DacStackReferenceWalker::GCEnumCallbackSOS(void*, __DPtr<Object>*, unsigned int, _DAC_SLOT_LOCATION) at daccess.cpp:8363))(__DPtr<__DPtr<Object> >, ScanContext*, unsigned int), void (*)(void*, __DPtr<Object>*, unsigned int, _DAC_SLOT_LOCATION)) at dacimpl.h:1994
    frame #9: 0x0000ffff7b4cf524 libmscordaccore.so`DacStackReferenceWalker::GetCount(this=0x0000000032e480d0, pCount=0x0000ffffc028acf4) at daccess.cpp:8297
    frame #10: 0x0000ffff7b829fa0 libsos.so`GetGCRefs(osID=4825, ppRefs=0x0000ffffc028ae18, pRefCnt=0x0000ffffc028ae24, ppErrors=0x0000000000000000, pErrCount=0x0000000000000000) at util.cpp:5907
    frame #11: 0x0000ffff7b7be488 libsos.so`GCRootImpl::PrintRootsOnThread(this=0x0000ffffc028b090, osThreadId=4825) at gcroot.cpp:731
    frame #12: 0x0000ffff7b7bc3d8 libsos.so`GCRootImpl::PrintRootsOnAllThreads(this=0x0000ffffc028b090) at gcroot.cpp:717
    frame #13: 0x0000ffff7b7bbcf0 libsos.so`GCRootImpl::PrintRootsForObject(this=0x0000ffffc028b090, target=281465514427248, all=false, noStacks=false) at gcroot.cpp:263
    frame #14: 0x0000ffff7b7f9eec libsos.so`::GCRoot(client=0x0000000032f65240, args=0x0000ffffc028b280) at strike.cpp:10711
    frame #15: 0x0000ffff0b14b498
    frame #16: 0x0000ffff0b14b29c
    frame #17: 0x0000ffff0b14ada8
    frame #18: 0x0000ffff824efd94 libcoreclr.so`CallDescrWorkerInternal at calldescrworkerarm64.S:72

[mikem8361]

No longer repros with the latest SOS from master and lldb 6.0.