libcflat: provide long division routines

The -nostdlib flag disables the driver from adding libclang_rt.*.a
during linking. Adding a specific library to the command line such as
libgcc then causes the linker to report unresolved symbols, because the
libraries that resolve those symbols aren't automatically added.

libgcc however is only needed for long division (64-bit by 64-bit).
Instead of linking the whole of it, implement the routines that are
needed.

Reported-by: Bill Wendling <morbo@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>