Basic support for RISCV vector instructions

[JakobSchaeffeler]

This PR adds basic support for RISC-V Vector instructions to the existing RISC-V backend.

Since there are no RISC-V Vector instructions that alter the control flow of a program this is quite straightforward. The only thing adapted in this PR is the scanner and the generator and the scanner to be able to handle vector instructions as regular non-control-flow instructions.
Right now the recognition for vector instructions is not very fine grained, but I could still change this if you think it is needed.

Additionally, to test this I added a plugin that counts the number of vector instructions executed in a program.

[JakobSchaeffeler]

Do you plan to merge this?
If you want I can also push more complex plugins that show that this is working.

[jkressel]

Hi @JakobSchaeffeler, thanks for submitting this and apologies for the delay, we plan to review asap.