eal: use pause only with SSE2

eal: use pause only with SSE2

The pause instruction is part of SSE2 extensions.
Note that some compilers define _mm_pause as "rep; nop" instead of "pause".
For compatible processors, they are equivalent.

http://www.intel.com/Assets/PDF/manual/325383.pdf:
"
When executing a spin-wait loop, a Pentium 4 or Intel Xeon processor suffers
a severe performance penalty when exiting the loop because it detects a
possible memory order violation.
The PAUSE instruction provides a hint to the processor that the code sequence
is a spin-wait loop. The processor uses this hint to avoid the memory order
violation in most situations, which greatly improves processor performance.
"

Signed-off-by: Intel