cbits: Work around GCC 4.5 SIMD stack alignment mismatch bug
cbits: Work around GCC 4.5 SIMD stack alignment mismatch bug

GCC pre-4.6 causes crashes in the SSE2 (and most likely others, when
stars are not aligned) implementation on Windows when calling
`set1_epi8_v`: when storing the result, the target memory is not 16-byte
aligned, causing a GPF/segfault. This could also occur with other
functions, but was only witnessed on Appveyor with `set1_epi8_v` on top
of the stack.

This commit adds a (hackish, and performance-impacting) work-around.

See: http://www.peterstock.co.uk/games/mingw_sse/
See: http://eigen.tuxfamily.org/dox/group__TopicWrongStackAlignment.html
See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838

NicolasT/reedsolomon,NicolasT/reedsolomon