linux/linkage_checker: remove gcc from undeclared_deps

This is causing warnings all over Homebrew/homebrew-core.

For example, there are dozens of warnings about this in the following CI
run:

    https://github.com/Homebrew/homebrew-core/runs/8116257943?check_suite_focus=true

Library/Homebrew/extend/os/linux/linkage_checker.rb

@@ -80,5 +80,7 @@ class LinkageChecker
     @unwanted_system_dylibs = @system_dylibs.reject do |s|
       SYSTEM_LIBRARY_ALLOWLIST.include? File.basename(s)
     end
+
+    @undeclared_deps.delete("gcc")
   end
 end