[tilegx] Need 64bit width for TileGX reg bit map.
[tilegx] Fix NameDictionaryLookupStub::GenerateNegativeLookup bug.

  * register bit should be with 64-bit width.
  * Multipush implicitly use "at" reg, so we should use "at2" to avoid
    reg collision.
[tilegx] We need 64-bit to contain our address.