work around g++ compiler code generation bug with -O3 and -Ofast by using -O2

Author: davidly

mr.sh

@@ -1 +1,4 @@
-g++ -flto -ggdb -Ofast -fno-builtin -D NDEBUG -I . ntvdm.cxx i8086.cxx -o ntvdm -fopenmp -static
+# this compiler: g++ (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
+# generates bad code using -O3 and -Ofast. I've yet to get to the bottom of this compiler bug.
+
+g++ -flto -ggdb -O2 -fno-builtin -D NDEBUG -I . ntvdm.cxx i8086.cxx -o ntvdm -fopenmp -static