US investment house Arnhold and S. Bleichroeder has bought a 5% stake in Austria's biggest beverage producer BBAG.   In an annoucement published in the Wiener Zeitung, BBAG said the investment house held 5.08% of the voting rights. The firm's majority owner is the brewer Brauerei Schwechat AG with a 66.79 percent stake while the remainder is free float.