ARM64: updated literal pool implementation.

Currently the literal pool implemetation is inherited from the arm 32-bit port
and it shares the same limitations: 4k of range and 1000 entries max. In arm64
the load literal has a 1MB range giving us more flexibility.

Immutable entries are now shared.

BUG=
R=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/338523005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21924 ce2b1a6d-e550-0410-aec6-3dcde31c8c00