Add int64 padding support for MirrorPad (#13907)

* Add int64 padding support for MirrorPad

This fix adds int64 padding support for `MirrorPad`.
In the `array_ops.cc` the `MirrorPad`/`MirrorPadGrad`
has been specified as supporting int64 padding. The related
kernels does not have the int64 padding registered though.
This fix adds the int64 padding support. This fix also adds
additional test cases for coverage.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update template for CPU and GPU support of int64 paddings.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add int64 padding support for MirrorPad

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Put eigen header first like before, just in case.