gnu: Use i686-linux bootstrap binaries on x86_64-linux.

This saves 4.4 MB on the installed Guix.

* gnu/packages/bootstrap/x86_64-linux: Remove directory.
* gnu/local.mk (bootstrap_x86_64_linuxdir)
(dist_bootstrap_x86_64_linux_DATA): Remove.
* gnu/packages.scm (search-bootstrap-binary): When SYSTEM is
"x86_64-linux", use "i686-linux" instead.

gnu/local.mk

@@ -1287,18 +1287,11 @@ MISC_DISTRO_FILES =				\
   %D%/packages/ld-wrapper.in
 
 bootstrapdir = $(guilemoduledir)/%D%/packages/bootstrap
-bootstrap_x86_64_linuxdir = $(bootstrapdir)/x86_64-linux
 bootstrap_i686_linuxdir = $(bootstrapdir)/i686-linux
 bootstrap_armhf_linuxdir = $(bootstrapdir)/armhf-linux
 bootstrap_aarch64_linuxdir = $(bootstrapdir)/aarch64-linux
 bootstrap_mips64el_linuxdir = $(bootstrapdir)/mips64el-linux
 
-dist_bootstrap_x86_64_linux_DATA =		\
-  %D%/packages/bootstrap/x86_64-linux/bash	\
-  %D%/packages/bootstrap/x86_64-linux/mkdir	\
-  %D%/packages/bootstrap/x86_64-linux/tar	\
-  %D%/packages/bootstrap/x86_64-linux/xz
-
 dist_bootstrap_i686_linux_DATA =		\
   %D%/packages/bootstrap/i686-linux/bash	\
   %D%/packages/bootstrap/i686-linux/mkdir	\

gnu/packages.scm

@@ -102,14 +102,18 @@ FILE-NAME found in %PATCH-PATH."
 (define (search-bootstrap-binary file-name system)
   "Search the bootstrap binary FILE-NAME for SYSTEM.  Raise an error if not
 found."
-  (or (search-path (%bootstrap-binaries-path)
-                   (string-append system "/" file-name))
-      (raise (condition
-              (&message
-               (message
-                (format #f (G_ "could not find bootstrap binary '~a' \
+  ;; On x86_64 always use the i686 binaries.
+  (let ((system (match system
+                  ("x86_64-linux" "i686-linux")
+                  (_ system))))
+    (or (search-path (%bootstrap-binaries-path)
+                     (string-append system "/" file-name))
+        (raise (condition
+                (&message
+                 (message
+                  (format #f (G_ "could not find bootstrap binary '~a' \
 for system '~a'")
-                        file-name system)))))))
+                          file-name system))))))))
 
 (define %distro-root-directory
   ;; Absolute file name of the module hierarchy.  Since (gnu packages …) might