Convert cord source files to valid C++ code

Convert cord source files to valid C++ code

Issue #206 (bdwgc).

* cord/cordbscs.c (Concatenation, Function, Generic): Define struct
outside union (i.e. the union just uses them).
* cord/cordbscs.c (CORD_cat_char_star, CORD_from_fn_inner,
CORD_substr_checked): Cast GC_MALLOC_ATOMIC() result to char*.
* cord/cordprnt.c (CORD_vsprintf): Likewise.
* cord/cordxtra.c (CORD_cat_char, CORD_to_char_star,
CORD_from_char_star, CORD_ec_flush_buf): Likewise.
* cord/cordbscs.c (CORD_substr_checked): Remove register keyword for
"result" local variable.