config: Stash known-good copies of config.guess|sub

Download config.guess|sub from
https://git.savannah.gnu.org/gitweb/?p=config.git (at hash
6faca61810d335c7837f320733fe8e15a1431fc2) in order to fix
#8410.

A future commit will install these files if they are newer than what
Autoconf installs.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

Author: jsquyres

config/from-savannah/README.md

@@ -0,0 +1,11 @@
+These files downloaded from
+https://git.savannah.gnu.org/gitweb/?p=config.git at git hash
+6faca61810d335c7837f320733fe8e15a1431fc2 on 26 Jan 2021.
+
+They were stashed here in the Open MPI repository in response to
+https://github.com/open-mpi/ompi/issues/8410, where it was determined
+that the responses from `config.*` installed by Autoconf were not
+sufficient for some modern platforms (e.g., Apple M1 Macs).
+
+`autogen.pl` will copy in these files if they are, in fact, newer than
+the corresponding files installed by Autoconf.