syscall: fix freebsd 64-bit test

64 bits is 8 bytes. Duh.

Change-Id: I991b359df6241889bdef13152f551af9db6e14c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/181557
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/syscall/syscall_freebsd.go

@@ -267,7 +267,7 @@ func Fstatfs(fd int, st *Statfs_t) (err error) {
 
 func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
 	if supportsABI(_ino64First) {
-		if unsafe.Sizeof(*basep) == 64 {
+		if unsafe.Sizeof(*basep) == 8 {
 			return getdirentries_freebsd12(fd, buf, (*uint64)(unsafe.Pointer(basep)))
 		}
 		// The freebsd12 syscall needs a 64-bit base. On 32-bit machines