Revert "Update ppcg_transform to output ppcg_scop to PTXGenerator"

This reverts commit ecdf871df78bca85211aa9e7f299ceff5311f900.
Rename *.c to *.cpp

So that we can use polly's data structure in ppcg. In the next step,
pet_stmt will be replaced with polly::ScopStmt. ppcg_option.c is
deleted because we never call any functions in it.