Replace the old --with-cxx-* configure options with a single --with-gcc-toolchain

Replace the old --with-cxx-* configure options with a single --with-gcc-toolchain
that just uses the new toolchain probing logic. This fixes linking with -m32 on
64 bit systems (the /32 dir was not being added to the search).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149651 91177308-0d34-0410-b5e6-96231b3b80d8