gnu: linux-libre: Update to 4.1.

* gnu/packages/linux.scm (linux-libre): Update to 4.1.
  [inputs]: Remove gcc-4.9, since it is now the default compiler.
* gnu/packages/linux-libre-i686.conf, gnu/packages/linux-libre-x86_64.conf:
  Update to 4.1 configs from <https://jxself.org/git/?p=kernel-configs.git>.