Prevent segfaults in SSE optimized Fletcher-4

In some cases, the compiler was not respecting the GNU aligned
attribute for stack variables in 35a76a0. This was resulting in
a segfault on CentOS 6.7 hosts using gcc 4.4.7-17. This issue
was fixed in gcc 4.6.

To prevent this from occurring, use unaligned loads and stores
for all stack and global memory references in the SSE optimized
Fletcher-4 code.

Disable zimport testing against master where this flaw exists:

TEST_ZIMPORT_VERSIONS="installed"

Signed-off-by: Tyler J. Stachecki <stachecki.tyler@gmail.com>
Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4862