syscall: use SYS_GETDENTS64 on linux/mips64{,le}

The getdents64 syscall is only available for mips64/mips64le starting
with Linux kernel 3.10. Since mips64le requires at least 4.8 according
to [1] (regarding #16848) using it should be fine.

[1] https://golang.org/wiki/MinimumRequirements

This CL changes the binary layout of type Dirent for mips64/mips64le,
but not the public API. But since the currently used layout doesn't
match the struct linux_dirent returned by the getdents syscall this
should be fine as well.

Fixes #23624

Change-Id: Iaa7306fa6e4442ad2fed41c60b37627a7314f117
Reviewed-on: https://go-review.googlesource.com/91055


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>