CI: Pin riscv-compliance repo to specific commit

Recently changes in the RISC-V compliance repo have caused our tests to
fail. That indicent highlighted the issue that we use the latest master
version of the compliance suite without pinning the version.

This PR fixes that by pinning to a specific version where our tests
still succeeded.

Upstream PR to fix actual failure: https://github.com/riscv/riscv-compliance/pull/92
Works around #608

azure-pipelines.yml

@@ -9,6 +9,7 @@ variables:
   VERILATOR_VERSION: 4.016
   VERILATOR_PATH: /opt/buildcache/verilator/$(VERILATOR_VERSION)
   RISCV_TOOLCHAIN_TAR_VERSION: 20190807-1
+  RISCV_COMPLIANCE_GIT_VERSION: 844c6660ef3f0d9b96957991109dfd80cc4938e2
 
 trigger:
   batch: true
@@ -125,6 +126,8 @@ jobs:
   - bash: |
       cd build
       git clone https://github.com/riscv/riscv-compliance.git
+      cd riscv-compliance
+      git checkout "$(RISCV_COMPLIANCE_GIT_VERSION)"
     displayName: Get RISC-V Compliance test suite
 
   - bash: |