arch/x86: add SSE floating-point to Intel64 subarch

This is a naive implementation which does "eager" context switching
for floating-point context, which, of course, introduces performance
concerns. Other approaches have security concerns, SMP implications,
and impact the x86 arch and Zephyr project as a whole. Discussion is
needed, so punting with the straightforward solution for now.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>