gcc: update to 9046071dddb0a9ee7e30e5edf27cfc53b5b9c238

Initializes the submodule

Unfortunately, Buildroot does not seem to use it:
https://stackoverflow.com/questions/54992977/how-to-select-an-override-srcdir-custom-source-code-for-gcc-when-building-buildr
so tracking it here just for fun for now. We don't clone on ./build by
default, so it should not impact anyone.

build

@@ -112,6 +112,8 @@ so looping over all of them would waste time.
             },
             submodules_shallow = {
                 'binutils-gdb',
+                # https://stackoverflow.com/questions/54992977/how-to-select-an-override-srcdir-source-for-gcc-when-building-buildroot
+                # 'gcc',
                 'glibc',
             },
             # https://buildroot.org/downloads/manual/manual.html#requirement