gnu: Use hurd-triplet? to check if GNU/Hurd.

* gnu/packages/make-bootstrap.scm (hurd-triplet?): Move it from here..
* gnu/packages/hurd.scm: ..to here. New exported procedure.
* gnu/packages/commencement.scm (glibc-final-with-bootstrap-bash)
  [arguments]: Replace string-match.
  [inputs]: Same.
* gnu/packages/cross-base.scm (cross-libc)[native-inputs]: Same.

gnu/packages/commencement.scm

@@ -474,7 +474,7 @@ the bootstrap environment."
                  (unsetenv "CPATH")
 
                  ;; Tell 'libpthread' where to find 'libihash' on Hurd systems.
-                 ,@(if (string-match "i586-gnu" (%current-system))
+                 ,@(if (hurd-triplet? (%current-system))
                        `((substitute* "libpthread/Makefile"
                            (("LDLIBS-pthread.so =.*")
                             (string-append "LDLIBS-pthread.so = "
@@ -499,7 +499,7 @@ the bootstrap environment."
         ,@%boot1-inputs
 
         ;; A native MiG is needed to build Glibc on Hurd.
-        ,@(if (string-match "i586-gnu" (%current-system))
+        ,@(if (hurd-triplet? (%current-system))
               `(("mig" ,mig-boot0))
               '())
 

gnu/packages/cross-base.scm

@@ -468,7 +468,7 @@ XBINUTILS and the cross tool chain."
 
           (native-inputs `(("cross-gcc" ,xgcc)
                            ("cross-binutils" ,xbinutils)
-                           ,@(if (string-match (or "i586-pc-gnu" "i586-gnu") target)
+                           ,@(if (hurd-triplet? target)
                                  `(("cross-mig"
                                     ,@(assoc-ref (package-native-inputs xheaders)
                                                  "cross-mig")))

gnu/packages/hurd.scm

@@ -28,7 +28,12 @@
   #:use-module (gnu packages bison)
   #:use-module (gnu packages perl)
   #:use-module (gnu packages base)
-  #:use-module (guix git-download))
+  #:use-module (guix git-download)
+  #:export (hurd-triplet?))
+
+(define (hurd-triplet? triplet)
+  (and (string-suffix? "-gnu" triplet)
+       (not (string-contains triplet "linux"))))
 
 (define-public gnumach-headers
   (package

gnu/packages/make-bootstrap.scm

@@ -337,10 +337,6 @@ for `sh' in $PATH, and without nscd, and with static NSS modules."
   ;; GNU libc's essential shared libraries, dynamic linker, and headers,
   ;; with all references to store directories stripped.  As a result,
   ;; libc.so is unusable and need to be patched for proper relocation.
-  (define (hurd-triplet? triplet)
-    (and (string-suffix? "-gnu" triplet)
-         (not (string-contains triplet "linux"))))
-
   (let ((glibc (glibc-for-bootstrap)))
     (package (inherit glibc)
       (name "glibc-stripped")