Add Zacas extension

[ved-rivos]

This PR adds tests for the Zacas extension that introduces the following instructions:

• amocas.w for RV32 and RV64
• amocas.d for RV32 and RV64
• amocas.q for RV64 only
  The specification is here: https://github.com/riscv/riscv-zacas
  The amocas instructions are emitted using a .word directive pending assembler support.

[aswaterman]

@ved-rivos did you make sure these can run on Spike?

[ved-rivos]

I have tested them on sail but not on spike.

[aswaterman]

Good enough for me.

[aswaterman]

Actually, good enough for me to believe the test cases are correct, but not good enough for being able to merge this :) We need it to be the case that executing make run in the isa directory succeeds. I think that means you need to add _zacas to the ISA strings in isa/Makefile so that the extension will be enabled for Spike. After doing that and making sure they run on Spike, we can merge.

[ved-rivos]

Thanks. These and all other tests pass on spike but the breakpoint.S test seems to be failing. I am not sure if I caused it (I most likely did not since I only added amocas tests :) ). I will update the PR after that debug that to be sure.

[ved-rivos]

Updated PR to extend the ISA string (breakpoint.S failure seems unrelated to addition of these tests).
FYI - SAIL PR: riscv/sail-riscv#285

[aswaterman]

Thanks, @ved-rivos. I’m sure you’re right, and I’ll look into the breakpoint test failure myself.

[ved-rivos]

@aswaterman - Found some time to look at this today. Seems like the test was not factoring in the case that when tcontrol is not implemented, mstatus.mie must be 1 for trigger with action=0 to match. I have sent PR #494 with a fix.