gnu: Refer to 'gcc' instead of 'gcc-4.9'.

* gnu/packages/commencement.scm (gcc-boot0, cross-gcc-wrapper,
  libstdc++, gcc-final): Refer to GCC instead of GCC-4.9.
* gnu/packages/cross-base.scm (%xgcc): New variable.
  (cross-gcc-arguments, cross-gcc): Refer to %XGCC instead of GCC-4.9.
* gnu/packages/llvm.scm (clang-from-llvm): Refer to GCC instead of GCC-4.9.
* gnu/packages/make-bootstrap.scm (package-with-relocatable-glibc,
  %gcc-static, %gcc-stripped): Likewise.