Update generate_gpu to use gen_ext

So we can use gen_ext to output the generated ast_node and gpu_prog.
Add two flags for GPGPU codegen(isl)

 - enable-polly-gpgpu-isl: enable polly GPGPU codegen.
 - polly-gpgpu-triple-isl: set the GPU triple string.
Add dump_ast_node debug option
Add print_ast_node_as_c_format function
Add an IslPTXGenerator member to isl NodeBuilder
Trigger the gpgpu codegen in IslCodeGeneration::runOnScop
Fix bug: should collct array and scopstmt info before access
Combine may_write, tagged_may_write with must_write and tagged_must_write

In ppcg, may_write means all the writes (polly may_writes + must_writes).
Add ppcg_print_guarded to get guard ast_node of the host node.
Update comment
Comment unused variable declaration
Set the same access ref_id to gpu_stmt and MemoryAccess
Add createForGPGPU function to generate the LLVM IR
Fix bug: move up isl_ast_expr_free(StmtExpr)

Include the free statement for if and if-not GPU_CODEGEN defined
Add #ifdef GPU_CODEGEN guard to changes of NodeBuilder
Fix a typo and update format
Add setGPUKernel function to set the current kernel
Add function addKernelSynchronizaton

By using this function we can add synchronization statement to kernel function.
Add createKernelDomain, createKernelCopy and createKernelSync
Get the current llvm::Value of grid ids and block ids