Add support for near labels.

Add support for near labels.

This change introduces near labels in the assembler, allowing us to
uptimize forward jumps (conditional and unconditional) if we can
guarantee that the jump is witin range -128 to +127.

I changed a large fractions of the existing Labels to NearLabels, and
left out cases where it was not immediately clear if it could be used
or not (not immediately clear means labels covering a large code
block, or used in function calls which we could potentially change to
accept near labels).

Review URL: http://codereview.chromium.org/3388004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5460 ce2b1a6d-e550-0410-aec6-3dcde31c8c00