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Fix race between calloc_explicitly_typed/push_complex_descriptor with lock
(fix of commits 4e020ef3c, 1e6c80be3)

Issue #449 (bdwgc).

Hold the allocation lock while writing a complex descriptor to the
object to ensure that the descriptor is seen by GC_array_mark_proc
as expected.

It should be possible to avoid locking by using the atomic operations
(with the release barrier in GC_calloc_explicitly_typed) but this
would need the acquire barrier in GC_array_mark_proc and its callee
(doing without the acquire barrier is tricky as GC_mark_some might be
invoked with the world running).

* typd_mlc.c [AO_HAVE_store] (GC_calloc_explicitly_typed): Do not use
AO_store operations.
* typd_mlc.c (GC_calloc_explicitly_typed): Remove volatile for lp
local variable; add comment; replace set_obj_descr() calls with direct
write to ((word*)op)[nwords-1]; wrap write to *lp and writing to
((word*)op)[nwords-1] into LOCK/UNLOCK.
* typd_mlc.c (get_complex_descr): Remove TODO item (but not
GC_ATTR_NO_SANITIZE_THREAD attribute).

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