[vector] Ensure stores to RO buffer throw exception

[Paul Sandoz]

Hi,

Please review this change that ensures a ReadOnlyBufferException is thrown for stores to a read-only ByteBuffer:

  http://cr.openjdk.java.net/~psandoz/panama/vector-load-store-ro-buffer/webrev/ <http://cr.openjdk.java.net/~psandoz/panama/vector-load-store-ro-buffer/webrev/>

Paul.