guix: pre time-machine bump changes (Windows)

Split out of #27897. This is some refactoring to the Windows Guix build
that facilitates bumping our Guix time-machine. Namely, avoiding
`package-with-extra-configure-variable`, which is non-functional in the
newer time-machine, see https://issues.guix.gnu.org/64436.

At the same time, consolidate our Windows GCC build into mingw-w64-base-gcc.
Rename `gcc-10-remap-guix-store.patch` to avoid changing it whenever GCC changes.

We move the old `building-on` inside `explicit-cross-configure`, so that
non-windows builds continue to work. Note that `explicit-cross-configure`
will be going away entirely (see #27897).

contrib/guix/manifest.scm

@@ -40,20 +40,6 @@ FILE-NAME found in ./patches relative to the current file."
       ((%patch-path (list (string-append (dirname (current-filename)) "/patches"))))
     (list (search-patch file-name) ...)))
 
-(define (make-ssp-fixed-gcc xgcc)
-  "Given a XGCC package, return a modified package that uses the SSP function
-from glibc instead of from libssp.so. Our `symbol-check' script will complain if
-we link against libssp.so, and thus will ensure that this works properly.
-
-Taken from:
-http://www.linuxfromscratch.org/hlfs/view/development/chapter05/gcc-pass1.html"
-  (package
-    (inherit xgcc)
-    (arguments
-     (substitute-keyword-arguments (package-arguments xgcc)
-       ((#:make-flags flags)
-        `(cons "gcc_cv_libc_provides_ssp=yes" ,flags))))))
-
 (define (make-gcc-rpath-link xgcc)
   "Given a XGCC package, return a modified package that replace each instance of
 -rpath in the default system spec that's inserted by Guix with -rpath-link"
@@ -71,9 +57,10 @@ http://www.linuxfromscratch.org/hlfs/view/development/chapter05/gcc-pass1.html"
                  (("-rpath=") "-rpath-link="))
                #t))))))))
 
-(define building-on (string-append (list-ref (string-split (%current-system) #\-) 0) "-guix-linux-gnu"))
+(define building-on (string-append "--build=" (list-ref (string-split (%current-system) #\-) 0) "-guix-linux-gnu"))
 
 (define (explicit-cross-configure package)
+  (define building-on (string-append (list-ref (string-split (%current-system) #\-) 0) "-guix-linux-gnu"))
   (package-with-extra-configure-variable package "--build" building-on))
 
 (define (make-cross-toolchain target
@@ -153,26 +140,20 @@ desirable for building Bitcoin Core release binaries."
                         base-libc
                         base-gcc))
 
-(define (make-gcc-with-pthreads gcc)
-  (package-with-extra-configure-variable
-    (package-with-extra-patches gcc
-      (search-our-patches "gcc-10-remap-guix-store.patch"))
-    "--enable-threads" "posix"))
-
-(define (make-mingw-w64-cross-gcc cross-gcc)
-  (package-with-extra-patches cross-gcc
-    (search-our-patches "vmov-alignment.patch"
+(define (gcc-mingw-patches gcc)
+  (package-with-extra-patches gcc
+    (search-our-patches "gcc-remap-guix-store.patch"
+                        "vmov-alignment.patch"
                         "gcc-broken-longjmp.patch")))
 
 (define (make-mingw-pthreads-cross-toolchain target)
   "Create a cross-compilation toolchain package for TARGET"
   (let* ((xbinutils (cross-binutils target))
          (pthreads-xlibc mingw-w64-x86_64-winpthreads)
-         (pthreads-xgcc (make-gcc-with-pthreads
-                         (cross-gcc target
-                                    #:xgcc (make-ssp-fixed-gcc (make-mingw-w64-cross-gcc base-gcc))
+         (pthreads-xgcc (cross-gcc target
+                                    #:xgcc (gcc-mingw-patches mingw-w64-base-gcc)
                                     #:xbinutils xbinutils
-                                    #:libc pthreads-xlibc))))
+                                    #:libc pthreads-xlibc)))
     ;; Define a meta-package that propagates the resulting XBINUTILS, XLIBC, and
     ;; XGCC
     (package
@@ -543,6 +524,22 @@ inspecting signatures in Mach-O binaries.")
       "--enable-stack-protector" "all")
       "--enable-bind-now" "yes"))
 
+(define-public mingw-w64-base-gcc
+  (package
+    (inherit base-gcc)
+    (arguments
+      (substitute-keyword-arguments (package-arguments base-gcc)
+        ((#:configure-flags flags)
+          `(append ,flags
+            ;; https://gcc.gnu.org/install/configure.html
+            (list "--enable-threads=posix",
+                  building-on)))
+        ((#:make-flags flags)
+          ;; Uses the SSP functions from glibc instead of from libssp.so.
+          ;; Our 'symbol-check' script will complain if we link against libssp.so,
+          ;; and thus will ensure that this works properly.
+          `(cons "gcc_cv_libc_provides_ssp=yes" ,flags))))))
+
 (define-public glibc-2.27
   (package
     (inherit glibc-2.31)