Set minimum size of byte vector to 4 on aarch64

[Qi Feng]

Hi,

This patch fixes a failure in javax/xml/crypto/dsig/GenerationTests.java
on aarch64. The failure is caused by the generation of size 2 ReplicateB,
in the meantime minimum ReplicateB size supported on aarch64 is 4.
Please help review it.

Webrev: http://cr.openjdk.java.net/~qfeng/panama/vectorapi/vector_min_size/webrev.00/

Thanks,
Qi