Fix for PR4192: fix the definition of int64_t on x86_64 Linux.

Note that I'm guessing that *BSD and Solaris do the same thing as Linux 
here, but it's quite possible I'm wrong; if the following testcase 
gives an error on x86-64 with gcc for any of those operating systems, please
tell me:

#include <stdint.h>

int64_t x; long x;

llvm-svn: 74583