gnu: gccgo: Reword synopsis and description.

Fixes <https://bugs.gnu.org/43303>.
Reported by Jeffrey Walton <noloader@gmail.com>.

* gnu/packages/gcc.scm: (gccgo-4.9): Add synopsis and description

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/gcc.scm

@@ -773,7 +773,13 @@ (define-public libgccjit
 
 
 (define-public gccgo-4.9
-  (custom-gcc gcc-4.9 "gccgo" '("go")
+  (custom-gcc (package
+                (inherit gcc-4.9)
+                (synopsis "Go frontend to GCC")
+                (description
+                 "This package is part of the GNU Compiler Collection and
+provides the GNU compiler for the Go programming language."))
+              "gccgo" '("go")
               %generic-search-paths
               ;; Suppress the separate "lib" output, because otherwise the
               ;; "lib" and "out" outputs would refer to each other, creating