gnu: cross-base: Distinguish cross Linux headers from native headers.

Commit c6d33a9 provided an incorrect fix, whereby the "linux-headers" key
would be used twice in %BUILD-INPUTS and the 'set-cross-path' phase would
refer to the first one of them, which happened to be the native headers, and
not the target headers.  This patch solves the problem by removing the
ambiguity.

* gnu/packages/cross-base.scm (cross-gcc-arguments)[set-cross-path]: Remove to
  "xlinux-headers" instead of "linux-headers".
  (cross-gcc): When LIBC is true, add explicit "xlinux-headers" input, taken
  from LIBC's propagated inputs.

gnu/packages/cross-base.scm

@@ -165,7 +165,7 @@ may be either a libc package or #f.)"
                   ;; Add the cross Linux headers to CROSS_CPATH, and remove them
                   ;; from CPATH.
                   (let ((libc  (assoc-ref inputs "libc"))
-                        (linux (assoc-ref inputs "linux-headers")))
+                        (linux (assoc-ref inputs "xlinux-headers")))
                     (define (cross? x)
                       ;; Return #t if X is a cross-libc or cross Linux.
                       (or (string-prefix? libc x)
@@ -244,6 +244,9 @@ GCC that does not target a libc; otherwise, target that libc."
                                (alist-delete "libc" %final-inputs))))
            (if libc
                `(("libc" ,libc)
+                 ("xlinux-headers"                ;the target headers
+                  ,@(assoc-ref (package-propagated-inputs libc)
+                               "linux-headers"))
                  ,@inputs)
                inputs))))