Linked Frame GC Race

[kavon]

Caught this in the benchmarks directory's parallel cml-merge when the message size was set very high.

==================
WARNING: ThreadSanitizer: data race (pid=3448)
  Read of size 8 at 0x7f78c4652ea8 by thread T5:
    #0 majorGCscanLINKFRAMEpointer ../../gc/major-gc-scan.c:66 (a.out+0x20437)
    #1 ScanMajorToSpace ../../gc/major-gc.c:503 (a.out+0x1e3a4)
    #2 PromoteObj ../../gc/major-gc.c:420 (a.out+0x1e000)
    #3 <null> <null> (a.out+0x4ab15)
    #4 IdleVProc ../../vproc/vproc.c:673 (a.out+0x3f950)
    #5 NewVProc ../../vproc/vproc.c:394 (a.out+0x3ecd5)
    #6 InitWithLocation ../../cpu/topology.c:106 (a.out+0x4a3f3)
    #7 <null> <null> (libtsan.so.0+0x296ad)

  Previous write of size 8 at 0x7f78c4652ea8 by thread T3:
    #0 ForwardObjMajor ../../gc/major-gc.c:52 (a.out+0x1cd17)
    #1 majorGCscanBITPATpointer ../../gc/major-gc-scan.c:148 (a.out+0x207b0)
    #2 ScanMajorToSpace ../../gc/major-gc.c:503 (a.out+0x1e3a4)
    #3 MajorGC ../../gc/major-gc.c:322 (a.out+0x1d8ad)
    #4 MinorGC ../../gc/minor-gc.c:527 (a.out+0x18822)
    #5 RunManticore ../../vproc/apply.c:115 (a.out+0x43d29)
    #6 IdleVProc ../../vproc/vproc.c:673 (a.out+0x3f950)
    #7 NewVProc ../../vproc/vproc.c:394 (a.out+0x3ecd5)
    #8 InitWithLocation ../../cpu/topology.c:106 (a.out+0x4a3f3)
    #9 <null> <null> (libtsan.so.0+0x296ad)

  Thread T5 (tid=3469, running) created by main thread at:
    #0 pthread_create <null> (libtsan.so.0+0x2bcee)
    #1 ThreadCreate ../../include/os-threads.h:30 (a.out+0x49918)
    #2 SpawnAt ../../cpu/topology.c:118 (a.out+0x4a49b)
    #3 VProcInit ../../vproc/vproc.c:218 (a.out+0x3df67)
    #4 main ../../misc/main.c:121 (a.out+0x414c5)

  Thread T3 (tid=3465, running) created by main thread at:
    #0 pthread_create <null> (libtsan.so.0+0x2bcee)
    #1 ThreadCreate ../../include/os-threads.h:30 (a.out+0x49918)
    #2 SpawnAt ../../cpu/topology.c:118 (a.out+0x4a49b)
    #3 VProcInit ../../vproc/vproc.c:218 (a.out+0x3df67)
    #4 main ../../misc/main.c:121 (a.out+0x414c5)