Remove use of isl_printer parameter in "generate" function

The original generate function is divided into 2 parts:
 - the new generate function: store the generated ast_node host tree
   in gen and return void.
 - ppcg_print_guard: print the host end codes, e.g. the outermost
   non-parallel loops and if guards, not implemented yet.
 - gen_free: do the cleanup staff for gpu_gen struct.

Add declaration to gpu.h.

Two functions call or use "generate" as callback are commented out.
 - generate_wrap
 - generate_gpu