remove gcc 3.x requirement from documentation

remove gcc 3.x requirement from documentation

This text is no longer accurate. After the patch is applied, the
generated version at http://www.nongnu.org/qemu/qemu-doc.html should be
regenerated.

This patch is also a candidate for the stable branch. (The URL above is
probably generated from the stable branch anyways, so maybe it goes
without saying.)

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>