Rename clang::driver::tools::linuxtools to clang::driver::tools::gnutools.

This is about the GNU Binutils' assembler and linker, so reflect that in the
name.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178272 91177308-0d34-0410-b5e6-96231b3b80d8