Fix encoding anonymous struct with multiple fields on 32-bit systems

[saracen]

This fixes another 32-bit edge case, which would result in the last field of an anonymous struct being duplicated for every other field within that same struct when encoded.

The fix requires a slice to be copied, which is only needed on 32-bit systems. An existing fix was done for fieldsDirect, but this expands to using the copied slice for fieldsSub too.

I've added a test case that captures the failure.

On Linux, executing GOARCH=386 go test -run TestEncodeAnonymousStruct without the changes done in encode.go results in:

--- FAIL: TestEncodeAnonymousStruct (0.00s)
    --- FAIL: TestEncodeAnonymousStruct/embedded_anonymous_multiple_fields (0.00s)
        encode_test.go:373:
            have:
                [Inner2]
                  N = 4

                [Inner2]
                  N = 4
            want:
                [Inner1]
                  N = 3

                [Inner2]
                  N = 4

With the changes, it works as expected.

[saracen]

@arp242 Thank you for merging. Do you know when there will be a tagged release with this fix?