ARM64: Clean up support for explicit literal load.

This is the first patch to improve literal pool handling in arm64. Cleans up
assembler/macro-assembler access to literal pools.

BUG=
R=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/318773009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21723 ce2b1a6d-e550-0410-aec6-3dcde31c8c00