Override -mpreferred-stack-boundary on x86_64

When using SSE instructions, not normally allowed in the
kernel, the stack must be aligned at 16 bytes.  Later
kernels (3.8.13, but not 3.5.7) try to set
-mpreferred-stack-boundary=3 on x86_64 arch, so force it.
Merge pull request #6 from zultron/ubc3-dev

Override -mpreferred-stack-boundary on x86_64