Update riscv-isa-sim to 2020-11-02 snapshot

• Switch to the official upstream, github.com/riscv/riscv-isa-sim.
• Remove emulators/riscv-fesvr, as it is now bundled with Spike.
• Drop patches.

Submitted by: lwhsu (earlier version)
Reviewed by: lwhsu
Approved by: lwhsu (ports, maintainer)
Differential Revision: https://reviews.freebsd.org/D27144