build-system/gnu: Change default locale to C.UTF-8.

This is a followup to 1cebc334a7.

* guix/build-system/gnu.scm (gnu-build, gnu-cross-build): #:locale now
defaults to "C.UTF-8".
* guix/build/gnu-build-system.scm (install-locale): Likewise.
* gnu/packages/linux.scm (util-linux)[arguments]: In ‘pre-check’ phase,
remove ‘invalid-multibyte’ modification so the test runs under C.UTF-8.

Change-Id: I9a4bfe564bcd8bbd0d57ba04568eb6b8020bd051

gnu/packages/linux.scm

@@ -2281,11 +2281,7 @@ (define-public util-linux
                      ;; Change the test to refer to the right file.
                      (substitute* "tests/ts/misc/mcookie"
                        (("/etc/services")
-                        services))
-
-                     ;; The C.UTF-8 locale does not exist in our libc.
-                     (substitute* "tests/ts/column/invalid-multibyte"
-                       (("C\\.UTF-8") "en_US.utf8")))))
+                        services)))))
                (add-before 'check 'disable-setarch-test
                  (lambda _
                    ;; The setarch tests are unreliable in QEMU's user-mode

guix/build-system/gnu.scm

@@ -361,7 +361,7 @@ (define* (gnu-build name inputs
                     (make-dynamic-linker-cache? #t)
                     (license-file-regexp %license-file-regexp)
                     (phases '%standard-phases)
-                    (locale "en_US.utf8")
+                    (locale "C.UTF-8")
                     (system (%current-system))
                     (build (nix-system->gnu-triplet system))
                     (imported-modules %default-gnu-imported-modules)
@@ -504,7 +504,7 @@ (define* (gnu-cross-build name
 
                           (license-file-regexp %license-file-regexp)
                           (phases '%standard-phases)
-                          (locale "en_US.utf8")
+                          (locale "C.UTF-8")
                           (system (%current-system))
                           (build (nix-system->gnu-triplet system))
                           (imported-modules %default-gnu-imported-modules)

guix/build/gnu-build-system.scm

@@ -123,7 +123,7 @@ (define native-input-directories
               native-search-paths)))
 
 (define* (install-locale #:key
-                         (locale "en_US.utf8")
+                         (locale "C.UTF-8")
                          (locale-category LC_ALL)
                          #:allow-other-keys)
   "Try to install LOCALE; emit a warning if that fails.  The main goal is to