ixgbe: use cold attribute

ixgbe: use cold attribute

As well as the fast-path functions in the rxtx code, there are also
functions which set up and tear down the descriptor rings. Since these
are not performance critical functions, there is no need to have them
extensively optimized, so we add __attribute__((cold)) to their
definitions. This has the side-effect of making debugging them easier as
the compiler does not optimize them as heavily, so more variables are
accessible by default in gdb.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>