gnu: Resolve derivation lint warnings.

Computing derivations for these systems (i686-gnu and riscv32-linux) fails
with an error like the following:

  could not find bootstrap binary 'tar' for system

* gnu/packages/mes.scm (mescc-tools)[supported-systems]: Remove riscv32-linux.
* guix/packages.scm (%hurd-system): Remove i686-gnu.

Signed-off-by: Christopher Baines <mail@cbaines.net>

gnu/packages/mes.scm

@@ -234,7 +234,7 @@ Guile.")
     (build-system gnu-build-system)
     (supported-systems '("i686-linux" "x86_64-linux"
                          "armhf-linux" "aarch64-linux"
-                         "riscv32-linux" "riscv64-linux"))
+                         "riscv64-linux"))
     (arguments
      `(#:make-flags (list (string-append "PREFIX=" (assoc-ref %outputs "out")))
        #:test-target "test"

guix/packages.scm

@@ -423,7 +423,7 @@ from forcing GEXP-PROMISE."
 
 (define %hurd-systems
   ;; The GNU/Hurd systems for which support is being developed.
-  '("i586-gnu" "i686-gnu"))
+  '("i586-gnu"))
 
 (define %cuirass-supported-systems
   ;; This is the list of system types for which build machines are available.