[tilegx] Fix data layout description glitch

  TileGX native support 64bit integer.

  Middle-end optimizer will use data layout info to decide whether
  it's beneficial to do some instruction combine.

  After this fix, some redundant type conversions are omitted, and
  llvm auto-vectorizer begin to get reasonable performance improvements.

  test-case:

    void test4(int* restrict a, int* restrict b, int k)
    {
      for (int i = 0; i < k; i++)
      {
        a[i] = a[i] + b[i];
      }
    }

  before fix:
               no auto-vec     autovec
    gcc-4.4.6  800 cycles      650
    clang      1400            2000

  after fix
               no auto-vec     autovec
    gcc-4.4.6  800 cycles      650
    clang      1000            900