Add comments explaining why our Itanium spinlock implementation is safe.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/aef5fe7efee5bde4abd618adbaf4c13f44ee59ab

Modified Files
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src/include/storage/s_lock.h |   32 ++++++++++++++++++++++++++++++--
1 files changed, 30 insertions(+), 2 deletions(-)

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