build-libssp: Fetch the libssp source from gcc's github mirror

The GCC svn repo is no longer available, and fetching the whole GCC
git repo at around 1 GB is a bit wasteful, when we just need libssp
(which is less than 1 MB), but github provides svn access to git repos,
allowing us to fetch just this subdirectory.

build-libssp.sh

@@ -37,7 +37,7 @@ export PATH="$PREFIX/bin:$PATH"
 : ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64}}
 
 if [ ! -d libssp ]; then
-    svn checkout -q svn://gcc.gnu.org/svn/gcc/tags/gcc_7_3_0_release/libssp
+    svn checkout -q https://github.com/gcc-mirror/gcc/tags/releases/gcc-7.3.0/libssp
 fi
 
 cp libssp-Makefile libssp/Makefile