syscall: allocate 64 bits of "basep" for Getdirentries

Recent crashes on 386 Darwin appear to be caused by this system call
smashing the stack. Phenomenology shows that allocating more data
here addresses the probem.
The guess is that since the actual system call is getdirentries64, 64 is
what we should allocate.

Should fix the darwin/386 build.

R=rsc
CC=golang-codereviews
https://golang.org/cl/53840043

src/pkg/syscall/syscall_bsd.go

@@ -64,8 +64,11 @@ func Setgroups(gids []int) (err error) {
 
 func ReadDirent(fd int, buf []byte) (n int, err error) {
 	// Final argument is (basep *uintptr) and the syscall doesn't take nil.
+	// 64 bits should be enough. (32 bits isn't even on 386). Since the
+	// actual system call is getdirentries64, 64 is a good guess.
 	// TODO(rsc): Can we use a single global basep for all calls?
-	return Getdirentries(fd, buf, new(uintptr))
+	var base = (*uintptr)(unsafe.Pointer(new(uint64)))
+	return Getdirentries(fd, buf, base)
 }
 
 // Wait status is 7 bits at bottom, either 0 (exited),