x86_64 - support for AVX instructions

 * CPU will be checked for XSAVE and AVX support on boot. If both are
   found, they will be enabled.

 * If enabled, the kernel will use the XSAVE and XRSTOR instructions to
   save and restore FPU, SSE and AVX registers.

Originally-Submitted-by: Adam Sakareassen (with modifications)
if: Remove IFF_POLLING from kernel space.

User space macro is kept for compatibility.  Adjust comment a little bit.
netgraph7/fec: Don't use IF_ macros on ifnet.if_snd