Fix decoding of arm struct stat64 by aarch64 strace.

Fix decoding of arm struct stat64 by aarch64 strace.

aarch64's uapi header files have a struct stat but no struct stat64.
To correctly decode a 32-bit process' s struct stat64 we need
HAVE_STAT64, but then the build fails because there is no struct stat64.
Luckily, the aarch64 struct stat is structurally equivalent to the arm
struct stat64, so we can just reuse that.

* file.c [AARCH64] (stat64): Define to stat.

Signed-off-by: Elliott Hughes <enh@google.com>