Added gcc -msse3 support

Added gcc -msse3 support

It is enabled when selecting x86-64-modern target, this gives
another nice speed up:

On a Core i5-2500 (3300 Mhz, Sandy Bridge):

64 bit download version: 1597151 n/s

-flto : 1659664 n/s

-flto -msse3: 1732344 n/s

Patch suggested by Tom Vijlbrief.

Also unify flto, popcount and msse3 optimization under "modern"
target, note that this can break the "modern" build on old gcc that
do not support -flto option: in this case update gcc ;-) or default
to the standard build.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>