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
The Array function must be looked up in the native context.

Platforms x64, a64, arm and mips had the bug that the array function was looked
up in the global context instead of the native context. Fix this, restoring a
weakened assert in hydrogen along the way (by the fix for crbug 347528, which
helped find this case).

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/184383003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19597 ce2b1a6d-e550-0410-aec6-3dcde31c8c00