syscalls: Define AT_SYMLINK_NOFOLLOW et al.

* guix/build/syscalls.scm (AT_FDCWD, AT_SYMLINK_NOFOLLOW, AT_REMOVEDIR)
(AT_SYMLINK_FOLLOW, AT_NO_AUTOMOUNT, AT_EMPTY_PATH): New variables.
* tests/syscalls.scm ("utime with AT_SYMLINK_NOFOLLOW"): New test.