guix: pass --disable-tm-clone-registry to base GCC

We don't need support for transactional memory.

> Disable TM clone registry in libgcc. It is enabled in libgcc by default.
> This option helps to reduce code size for embedded targets which do
> not use transactional memory.

https://gcc.gnu.org/install/configure.html

contrib/guix/manifest_build.scm

@@ -199,6 +199,7 @@ chain for " target " development."))
                   "--disable-libsanitizer"
                   "--disable-lto"
                   "--disable-nls"
+                  "--disable-tm-clone-registry"
                   #$building-on)))
         ((#:phases phases)
           #~(modify-phases #$phases