A64: fix cctest/test-assembler-a64

The simulator now deletes its decoder in its dtor. Therefore, we must
always allocate the decoder on the heap.

BUG=none
R=ulan@chromium.org, jacob.bramley@arm.com
LOG=n

Review URL: https://codereview.chromium.org/183893005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19596 ce2b1a6d-e550-0410-aec6-3dcde31c8c00