guix: use gcc-12 to compile winpthreads

Currently, winpthreads is compiled with GCC 11, when we want to be using
GCC 12 for all compilation.

Author: fanquake

contrib/guix/manifest.scm

@@ -131,7 +131,10 @@ desirable for building Bitcoin Core release binaries."
 (define (make-mingw-pthreads-cross-toolchain target)
   "Create a cross-compilation toolchain package for TARGET"
   (let* ((xbinutils (binutils-mingw-patches (cross-binutils target)))
-         (pthreads-xlibc mingw-w64-x86_64-winpthreads)
+         (machine (substring target 0 (string-index target #\-)))
+         (pthreads-xlibc (make-mingw-w64 machine
+                                         #:xgcc (cross-gcc target #:xgcc (gcc-mingw-patches base-gcc))
+                                         #:with-winpthreads? #t))
          (pthreads-xgcc (cross-gcc target
                                     #:xgcc (gcc-mingw-patches mingw-w64-base-gcc)
                                     #:xbinutils xbinutils