[amd-spirv-backend][Buildbot] Create buildbots to test toolchain

[lamb-j]

To test the new SPIRV-backend-based implementation (and AMDGPU + SPIRV in general), we should create a downstream buildbot that tests the following:

• LLVM build and LIT spirv-related tests
• Comgr build and LIT spirv-related tests
• HIP build spirv-related tests

Once these are stable, we can investigate adding the following as well:

• HeCBench
• Babelstream
• OCL Conformance tests

Finally, we can also create an upstream buildbot, likely with a reduced set of tests, to catch issues before they hit the downstream branches.