[tilegx] Fix bfextu encoding bug.

  "BFSTART_X0 | BFSTART_X0" should be "BFSTART_X0 | BFEND_X0".
[tilegx] "1 << r" should be "1L << r" for >32 registers.
[tilegx] Fix TileGX special stack zone issue.

  When calling from JS code to C/C++ code, we need to reserve
16bytes on the bottom of the stack to comply with our GCC ABI.