Hi Guix!

I've been getting errors while running `guix pull' on an aarch64 system,
during the final guix-package-cache step:

--8<---------------cut here---------------start------------->8---
(repl-version 0 1 1)
Generating package cache for '/gnu/store/m8in1imi93snq711d7568dj9hlrx4diz-profile'...

Backtrace:
In ice-9/boot-9.scm:
  1747:15 19 (with-exception-handler #<procedure af1570 at ice-9/bo?> ?)
  1752:10 18 (with-exception-handler _ _ #:unwind? _ # _)
In guix/repl.scm:
    99:21 17 (_)
In unknown file:
          16 (_ #<procedure 82fd00 at guix/repl.scm:100:25 ()> #<pr?> ?)
          15 (primitive-load "/gnu/store/3x6g541ixbmdjav4ky6dp1ryj4l?")
In ice-9/boot-9.scm:
  1752:10 14 (with-exception-handler _ _ #:unwind? _ # _)
In gnu/packages.scm:
   438:11 13 (generate-package-cache _)
In srfi/srfi-1.scm:
   460:18 12 (fold #<procedure expand-cache expr> _ _)
In gnu/packages.scm:
    390:9 11 (expand-cache . _)
In guix/packages.scm:
  1317:17 10 (supported-package? #<package linux-libre@4.14.300 gnu?> ?)
In guix/memoization.scm:
    101:0  9 (_ #<hash-table 31605e0 13974/28099> #<package linux-l?> ?)
In guix/packages.scm:
  1295:37  8 (_)
  1555:16  7 (package->bag _ _ _ #:graft? _)
  1660:43  6 (thunk)
In gnu/packages/linux.scm:
   986:37  5 (arguments #<package linux-libre@4.14.300 gnu/packages/?>)
In guix/gexp.scm:
   460:52  4 (%local-file #f #<promise #<procedure 4df2660 at gnu/p?> ?)
In unknown file:
           3 (basename #f #<undefined>)
In ice-9/boot-9.scm:
  1685:16  2 (raise-exception _ #:continuable? _)
  1780:13  1 (_ #<&compound-exception components: (#<&assertion-fail?>)
In unknown file:
           0 (backtrace #<undefined>)

(exception wrong-type-arg (value "scm_to_utf8_stringn") (value "Wrong type argument in position ~A (expecting ~A): ~S") (value (1 "string" #f)) (value (#f)))
--8<---------------cut here---------------end--------------->8---

I was able to decipher the backtrace to *maybe* put together a fix, but
I'm unsure why the problem started. My best guess is that it started
with commit dfc6957a5af7d179d4618eb19d4f555c519bc6f2, even though I
can't find where the issue actually is, it looks fine to me!

What seems to happen is that the `kernel-config' function now receive an
`arch' argument for an architecture that isn't actually supported by
that kernel, as is the case for linux-libre@4.14.300.  And, correctly,
the function should not expect to ever get such arch value to begin
with, so we get a `(local-file #f)'.

--8<---------------cut here---------------start------------->8---
(define* (kernel-config arch #:key variant)
  "Return a file-like object of the Linux-Libre build configuration file for
ARCH and optionally VARIANT, or #f if there is no such configuration."
  (let* ((name (string-append (if variant (string-append variant "-") "")
                              (if (string=? "i386" arch) "i686" arch) ".conf"))
         (file (string-append "linux-libre/" name)))
    (local-file (search-auxiliary-file file))))
--8<---------------cut here---------------end--------------->8---

I think it's fair for that function expect the arch to be valid (why
would you ask the config for an unsupported arch?).

I think it should be possible to fix this by checking the arch is
supported at the call site: