Fix apparent typo, where CONFIG_64_BIT should read CONFIG_64BIT.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>