Remove void pointer arithmetics in system_allocator.c

We replace instances of void* w/ char* arithmetics.

This allows system_allocator.c to be compiled with MSVC.

Also we introduce a struct to represent the size_t header.

tools/windows/hammer_lib_src_list

@@ -7,6 +7,7 @@ cfgrammar.c
 desugar.c 
 glue.c 
 hammer.c 
+system_allocator.c
 parsers/action.c 
 parsers/and.c 
 parsers/attr_bool.c