c++: Fix coroutines.cc build for nvptx-none target

c++: Fix coroutines.cc build for nvptx-none target

When building offloading cross-compiler from x86_64-linux to nvptx-none, the
build fails with:
../../gcc/cp/coroutines.cc: In function 'tree_node* get_fn_local_identifier(tree, const char*)':
../../gcc/cp/coroutines.cc:2255:12: error: expected ';' before 'char'
 2255 |   sep = "$"
      |            ^
      |            ;
......
 2262 |   char *an;
      |   ~~~~

2020-01-18  Jakub Jelinek  <jakub@redhat.com>

	* coroutines.cc (get_fn_local_identifier): Fix NO_DOT_IN_LABEL
	but non-NO_DOLLAR_IN_LABEL case build.