gcc: Fix style [Linux]
gcc: Fix style [Linux]

* Stable: Please use https://
* 'fails_with :llvm' is now a no-op so should be removed
* Don't need 'FileUtils.' before mv.
* Use the `mv` Ruby method instead of `system "mv `

maxim-belkin/homebrew-core,Linuxbrew/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core,rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core