pgsql: Fix issues in e8fdbd58fe.

Fix issues in e8fdbd58fe.

When the 64bit atomics simulation is in use, we can't necessarily
guarantee the correct alignment of the atomics due to lack of compiler
support for doing so- that's fine from a safety perspective, because
everything is protected by a lock, but we asserted the alignment in
all cases. Weaken them. Per complaint from Alvaro Herrera.

My #ifdefery for PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY wasn't
sufficient. Fix that. Per complaint from Alexander Korotkov.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f13a9121f9822eafe05cc3178bf046155a248173

Modified Files
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src/include/port/atomics.h | 29 ++++++++++++++++++++--
src/include/port/atomics/generic.h | 49 +++++++++++++++++++++++++++-----------
2 files changed, 62 insertions(+), 16 deletions(-)