Change linux faccessat syscall definition to match actual linux one.

The AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are actually implemented
within the glibc wrapper function for faccessat(). If either of these
flags are specified, then the wrapper function employs fstatat() to
determine access permissions.

Differential Revision: https://reviews.freebsd.org/D1078
Reviewed by: trasz