Apple, Amazon and Google have joined the bidding on Toshiba’s NAND flash memory production division, Reuters reports. According to the Yomiuri Shimbun daily, Apple’s bid is still unknown, but other firms and companies have offered around $18 billion for the unit. A source with knowledge of the sale said about 10 bidders have expressed an interest so far, including Western Digital and U.S. chipmaker Broadcom. It’s unclear what Apple’s…