build-libssp: Stop building the real libssp from GCC

As mingw-w64 now provides these routines, we no longer need to build
the original library from GCC.

As building with -fstack-protector-strong still links with -lssp
(and -lssp_nonshared), we still need to provide libraries with these
names, but they can be empty dummy libraries.

build-all.sh

@@ -96,5 +96,5 @@ fi
 ./build-libcxx.sh $PREFIX $CFGUARD_ARGS
 ./build-mingw-w64-libraries.sh $PREFIX $CFGUARD_ARGS
 ./build-compiler-rt.sh $PREFIX --build-sanitizers # CFGUARD_ARGS intentionally omitted
-./build-libssp.sh $PREFIX $CFGUARD_ARGS
+./build-libssp.sh $PREFIX # CFGUARD_ARGS not used
 ./build-openmp.sh $PREFIX $CFGUARD_ARGS