mk: use -march option with recent Intel processors names

mk: use -march option with recent Intel processors names

The GCC 4.9 -march option supports the intel code names for processors,
for example -march=silvermont, -march=broadwell.
The RTE_MACHINE config flag can be used to pass code name to
the compiler as -march flag.

Release notes is updated.

Linux and FreeBSD getting started guides are updated with recommended
gcc version as 4.9 and above.

Some of the gmake command examples in sample application guide and driver
guides are updated with gcc version as 4.9.

Signed-off-by: Reshma Pattan <reshma.pattan@intel.com>