gnu: Use gcc-14, gcc-toolchain-14 on the 64bit Hurd.

This enables building packages in a 64bit childhurd.  It will not enable
offloading to a childhurd.

* guix/utils.scm (host-hurd?, host-x86-64, host-hurd64?): New procedures.
* gnu/packages/commencement.scm (gcc-toolchain): Use them to determine if the
host is a 64bit Hurd, and use gcc-toolchain-14.
* gnu/packages/gcc.scm (gcc): Likewise, to use gcc-14.

Change-Id: I55e43fdc61e3ea5fc13065fc7ca854b951c94930

gnu/packages/commencement.scm

@@ -3642,7 +3642,9 @@ (define-public gcc-toolchain-14
 
 ;; The default GCC
 (define-public gcc-toolchain
-  gcc-toolchain-11)
+  (if (host-hurd64?)
+      gcc-toolchain-14
+      gcc-toolchain-11))
 
 (define-public gcc-toolchain-aka-gcc
   ;; It's natural for users to try "guix install gcc".  This package

gnu/packages/gcc.scm

@@ -858,7 +858,10 @@ (define-public gcc-14
 
 ;; Note: When changing the default gcc version, update
 ;;       the gcc-toolchain-* definitions.
-(define-public gcc gcc-11)
+(define-public gcc
+  (if (host-hurd64?)
+      gcc-14
+      gcc-11))
 
 
 ;;;

guix/utils.scm

@@ -96,6 +96,9 @@ (define-module (guix utils)
             system-hurd?
             target-hurd64?
             system-hurd64?
+            host-hurd?
+            host-hurd64?
+            host-x86-64?
             target-mingw?
             target-x86-32?
             target-x86-64?
@@ -729,6 +732,22 @@ (define* (system-hurd64?)
   (and (system-hurd?)
        (target-64bit? (%current-system))))
 
+(define (host-hurd?)
+  "Are we running on a Hurd system?  This is almost never the right function,
+use target-hurd? or system-hurd? instead."
+  (equal? (utsname:sysname (uname)) "GNU"))
+
+(define (host-x86-64?)
+  "Are we running on a x86_64 system?  This is almost never the right
+function, use target-x86-64? or system-x86-64? instead."
+  (equal? (utsname:machine (uname)) "x86_64"))
+
+(define (host-hurd64?)
+  "Are we running on a 64bit Hurd?  This is almost never the right
+function, use target-hurd64? or system-hurd64? instead."
+  (and (host-hurd?)
+       (host-x86-64?)))
+
 (define* (target-mingw? #:optional (target (%current-target-system)))
   "Is the operating system of TARGET Windows?"
   (and target