Update config.guess to add riscv64 architecture support for Privoxy

Hi,
I am packaging this project for Arch Linux RISC-V and I noticed that *config.guess *script this project* *use is too old to recognize riscv64 arch. This script should be updated to support riscv64 arch.
To update this script manually while packaging, I ran this command after fetching the tarball:
autoheader && autoreconf -fi && autoupdate -f && ./configure
then this problem should be fixed.
Thx.