cpustrs2.txt: Mention VGA modes and minor formatting change.
Added "vga=ext" options to 'linpack' and 'systester' boot entries.
These 2 programs can produce output of >25 lines simutaneously. Some of the lines can go off the screen quickly, before the user can read. Therefore use extended VGA mode for the 2 programs. However, the font in 80x50 is ugly and so the mode is not enabled for the rest.