pgsql: Handle unaligned SerializeSnapshot() buffer.

Handle unaligned SerializeSnapshot() buffer.

Likewise in RestoreSnapshot(). Do so by copying between the user buffer
and a stack buffer of known alignment. Back-patch to 9.6, where this
last applies cleanly. In master, the select_parallel test dies with
SIGBUS on "Oracle Solaris 10 1/13 s10s_u11wos_24a SPARC", building
32-bit with gcc 4.9.2. In 9.6 and 9.5, the buffers in question happen
to be sufficiently-aligned, and this change is mere insurance against
future 9.6 changes or extension code compromising that.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/7f3112135eb67e5df56cd34b8ce662bf6a2390e9

Modified Files
--------------
src/backend/utils/time/snapmgr.c | 76 +++++++++++++++++++++-------------------
1 file changed, 40 insertions(+), 36 deletions(-)