Local variables can also be relied on the be 64-bit aligned, since
they will be escaped to the heap if used with any atomic operations.

Also, allocated arrays are also aligned, just like structs and slices.

Fixes #18955.

Change-Id: I8a1897f6ff78922c8bfcf20d6eb4bcb17a70ba2d
Reviewed-on: https://go-review.googlesource.com/48112Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>