Update opcodes for custom instructions

It looks like there was a mismatch between some of the custom instructions and their encodings. I've attempted to make the instructions match the formats described in the ISA spec.

I'll also have pull requests for riscv-isa-sim, riscv-pk, and riscv-test-env for encoding.h.

I was planning to submit a pull request for riscv-gcc, but it looks like the riscv-opc.h files haven't been updated in a while and I was getting a big diff.

Summary of changes:

* Adjust encodings to match each instruction name
  e.g., custom.rs1.rs2 should match the S-type format
* Provide examples for R, I, S, SB, U, and UJ type instructions.
* Remove custom*.rs1 instructions, since they do not map to an
  existing format