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.

Dockerfile

@@ -63,8 +63,8 @@ RUN ./build-llvm.sh $TOOLCHAIN_PREFIX && \
     rm -rf /build/*
 
 # Build libssp
-COPY build-libssp.sh libssp-Makefile ./
-RUN ./build-libssp.sh $TOOLCHAIN_PREFIX $CFGUARD_ARGS && \
+COPY build-libssp.sh ./
+RUN ./build-libssp.sh $TOOLCHAIN_PREFIX && \
     rm -rf /build/*
 
 ENV PATH=$TOOLCHAIN_PREFIX/bin:$PATH