gnu: gdb: Build with a 32-bit MIG when targeting i586-pc-gnu.

Fixes a bug whereby GDB cross-compiled for i586-pc-gnu would be broken:

  (gdb) r
  Starting program: /gnu/store/…
  Handling event, msgid = 72: (ipc/mig) server type check failure

* gnu/packages/gdb.scm (gdb-9.1)[native-inputs]: Use MIG/32-BIT instead
of MIG.

gnu/packages/gdb.scm

@@ -116,7 +116,13 @@
       `(("texinfo" ,texinfo)
         ("dejagnu" ,dejagnu)
         ("pkg-config" ,pkg-config)
-        ,@(if (hurd-target?) `(("mig" ,mig)) '())))
+        ,@(if (hurd-target?)
+              ;; When cross-compiling from x86_64-linux, make sure to use a
+              ;; 32-bit MiG because we assume target i586-pc-gnu.
+              `(("mig" ,(if (%current-target-system)
+                            mig/32-bit
+                            mig)))
+              '())))
     (home-page "https://www.gnu.org/software/gdb/")
     (synopsis "The GNU debugger")
     (description