Build libssp from gcc

gcc/libssp's configure script runs checks for flags that clang doesn't
implement. We actually just need to set a few HAVE defines and compile
the .c sources, so just check out this subdirectory using svn and build
it using a custom makefile.

.gitignore

@@ -4,3 +4,4 @@ compiler-rt
 libunwind
 libcxxabi
 libcxx
+libssp