Use standalone private macro to guard against ptr_t redefinition

Use standalone private macro to guard against ptr_t redefinition
(code refactoring)

* include/private/gc_priv.h (ptr_t): Define only if PTR_T_DEFINED is
not defined (previously).
* include/private/gcconfig.h (ptr_t): Likewise.
* include/private/gc_priv.h (PTR_T_DEFINED): Define macro.
* include/private/gcconfig.h (PTR_T_DEFINED): Likewise.
* include/private/gcconfig.h (ptr_t): Change type to char*; remove
void comments.
* include/private/gcconfig.h: Include stddef.h only if sony_news is
not defined (like in gc_priv.h).