Fix GCC warnings about wrong types in va_arg() calls

native.c:172:42: warning: 'bool' is promoted to 'int' when passed through '...' [enabled by default]
           params[i] = (cell)va_arg(args, bool);
                                          ^
native.c:172:42: note: (so you should pass 'int' not 'bool' to 'va_arg')
native.c:172:42: note: if this code is reached, the program will abort
native.c:175:38: warning: 'float' is promoted to 'double' when passed through '...' [enabled by default]
           float value = va_arg(args, float);
                                      ^
native.c:175:38: note: if this code is reached, the program will abort