erasure-code: do not attempt to compile SSE4 on i386

SSE4 are only not availabe on older CPUs. Although the compiler could
probably generate the code, there is no point in doing so. The SSE4.1,
SSE4.2 and PCLMUL cpu features are only tested if the target CPU is
AMD64 or x86_64.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1563 from dachary/wip-sse-i386

erasure-code: do not attempt to compile SSE4 on i386

Reviewed-by: Sage Weil <sage@inktank.com>