Replace C++ style comments to C ones, remove commented out code (extra)

Replace C++ style comments to C ones, remove commented out code (extra)

* extra/AmigaOS.c (GC_amiga_get_mem, GC_amiga_allocwrapper_any,
GC_amiga_allocwrapper_fast): Replace the style of comments from C++
to C.
* extra/MacOS.c (CodeZeroPtr, GC_MacTemporaryNewPtr): Likewise.
* extra/Mac_files/MacOS_config.h: Likewise.
* extra/AmigaOS.c (GC_amiga_get_mem, GC_amiga_rec_alloc,
GC_amiga_allocwrapper_any, GC_amiga_allocwrapper_fast): Remove
commented out code.
* extra/Mac_files/MacOS_config.h: Likewise.