Make NextCodeLink pointer 8-byte aligned after GC pointer updating uses atomics.

Make NextCodeLink pointer 8-byte aligned after GC pointer updating uses atomics.

After r24737 pointer updating is done by atomic operations which strictly
require target address to be 8-byte alignment on MIPS64.

The alignment is broken on 64-bit arches because InstructionSize is first field
and has Int size.

Order of fields in object layout are changed to make kNextCodeLinkOffset
divisible by 8. The size of code object header remains the same.

TEST=cctest/test-debug/* on MIPS64
BUG=
R=jkummerow@chromium.org, paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/682673002

Cr-Commit-Position: refs/heads/master@{#24914}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24914 ce2b1a6d-e550-0410-aec6-3dcde31c8c00