Move libssp into musl out of GCC

The motivations for this are complicated, but on musl systems, musl
will use its own libssp implementation, so GCC's libssp is not required.

Not to mention that GCC's libssp implementation is questionable at best.

This is the approach taken by the two major musl distributions - Alpine
Linux and Void Linux.

sysc/gcc-10.4.0/gcc-10.4.0.sh

@@ -95,5 +95,6 @@ src_configure() {
         --disable-sjlj-exceptions \
         --disable-multilib \
         --enable-threads=posix \
-        --disable-libsanitizer
+        --disable-libsanitizer \
+        --disable-libssp
 }