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

Commit c6d33a9d 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.