Don't use openat64 in fcntl2.h under NaCl

The NaCl build used 64-bit symbols by default and
openat64 is therefore not available.  fcntl2.h
is only used when _FORTITY_SOURCE > 2.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4241
TEST= local verification + trybots
R=mcgrathr@chromium.org

Review URL: https://codereview.chromium.org/1337243004 .
1 file changed