Move 'nix-system->gnu-triplet' to (guix utils) and export it.

* gnu/packages/commencement.scm (nix-system->gnu-triplet): Move to...
* guix/utils.scm (nix-system->gnu-triplet): ... here.  Fix docstring typo.

gnu/packages/commencement.scm

@@ -122,19 +122,6 @@
     ("file" ,file-boot0)
     ,@%bootstrap-inputs))
 
-(define* (nix-system->gnu-triplet
-          #:optional (system (%current-system)) (vendor "unknown"))
-  "Return an a guess of the GNU triplet corresponding to Nix system
-identifier SYSTEM."
-  (let* ((dash (string-index system #\-))
-         (arch (substring system 0 dash))
-         (os   (substring system (+ 1 dash))))
-    (string-append arch
-                   "-" vendor "-"
-                   (if (string=? os "linux")
-                       "linux-gnu"
-                       os))))
-
 (define* (boot-triplet #:optional (system (%current-system)))
   ;; Return the triplet used to create the cross toolchain needed in the
   ;; first bootstrapping stage.

guix/utils.scm

@@ -61,6 +61,7 @@
             location-column
             source-properties->location
 
+            nix-system->gnu-triplet
             gnu-triplet->nix-system
             %current-system
             %current-target-system
@@ -476,6 +477,19 @@ previous value of the keyword argument."
          (()
           (reverse before)))))))
 
+(define* (nix-system->gnu-triplet
+          #:optional (system (%current-system)) (vendor "unknown"))
+  "Return a guess of the GNU triplet corresponding to Nix system
+identifier SYSTEM."
+  (let* ((dash (string-index system #\-))
+         (arch (substring system 0 dash))
+         (os   (substring system (+ 1 dash))))
+    (string-append arch
+                   "-" vendor "-"
+                   (if (string=? os "linux")
+                       "linux-gnu"
+                       os))))
+
 (define (gnu-triplet->nix-system triplet)
   "Return the Nix system type corresponding to TRIPLET, a GNU triplet as
 returned by `config.guess'."