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.

.dockerignore

@@ -1,7 +1,6 @@
 .git
 llvm-project
 mingw-w64
-libssp
 lldb-mi
 make-*
 libffi