gnu: avr-gcc: Avoid referencing gcc from the top level.

Fixes <https://issues.guix.gnu.org/65716>.

* gnu/packages/avr.scm (avr-gcc): Do not explicitly refer to 'gcc'.  The
default value of the argument uses the identifier-syntax trick to delay
evaluation of gcc.

gnu/packages/avr.scm

@@ -46,7 +46,7 @@
     (name "avr-binutils")))
 
 (define avr-gcc
-  (let ((xgcc (cross-gcc "avr" #:xgcc gcc #:xbinutils avr-binutils)))
+  (let ((xgcc (cross-gcc "avr" #:xbinutils avr-binutils)))
     (package
       (inherit xgcc)
       (name "avr-gcc")