[tilegx] Fix various code-stub & assembler issues.

  these fixes let snapshot generation process could run until SEGSEGV.
  we need to enable v8 builtin debug system for tilegx, so that the
  error could be traced.
[tilegx] Support tilegx.debug build.
[tilegx] Fix sign extension bug.
[tilegx] Implement register based call (jr).
[tilegx] Implement Addu.
[tilegx] Integrate standalone disassembler into tilegx v8.
[tilegx] Add missing backend .cc files.