Fix 'variable assigned a value that is never used' cppcheck style warnings

Fix 'variable assigned a value that is never used' cppcheck style warnings

* dyn_load.c [USE_PROC_FOR_LIBRARIES] (GC_register_map_entries): Remove
datastart local variable.
* extra/AmigaOS.c [GC_AMIGA_DS] (GC_register_data_segments): Remove
num local variable.
* extra/MacOS.c (firstTime): Remove.
* extra/MacOS.c (GC_MacTemporaryNewPtr): Do not define firstTime if
SHARED_LIBRARY_BUILD.
* extra/MacOS.c (GC_MacFreeTemporaryMemory): Do not define (and update)
totalMemoryUsed if SHARED_LIBRARY_BUILD.
* extra/msvc_dbg.c (GetDescriptionFromAddress): Do not assign size
variable when its value no longer used.