[llvm][tools] Improve llvm-gpu-loader checks

[adurang]

When the file format is incorrect, or the platform or devices are not properly initialized llvm-gpu-loader follows corrupt pointers which result in hard to debug crashes.

This improves the checks to avoid such situations.

[jhuber6]

This was an intentional fall-through. When we can't determine it from the ELF flags we let olInit initialize every architecture.

[adurang]

I'm not sure I see the benefit of doing that. The only other format we support right now is the bitcode and we could add a check here. I think it's better to error here with a more meaningful message than later with a "no device found" error which was due to an unsupported format.

[jhuber6]

This was an intentional fall-through. When we can't determine it from the ELF flag we let olInit initialize every plugin. If it fails to find a compatible device that will fail later.

[adurang]

ok, but by the same token shouldn't we let fall-through ELFs that are not EM_AMDGPU or EM_CUDA instead of erring out?

[llvm-ci]

LLVM Buildbot has detected a new failure on builder ppc64le-mlir-rhel-clang running on ppc64le-mlir-rhel-test while building llvm at step 3 "clean-build-dir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/129/builds/40418

Here is the relevant piece of the build log for the reference

Step 3 (clean-build-dir) failure: Delete failed. (failure) (timed out)
Step 6 (test-build-check-mlir-build-only-check-mlir) failure: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
...
PASS: MLIR :: Pass/pipeline-options-parsing.mlir (3932 of 3942)
PASS: MLIR-Unit :: IR/./MLIRIRTests/0/142 (3933 of 3942)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/11/22 (3934 of 3942)
PASS: MLIR :: mlir-reduce/simple-test.mlir (3935 of 3942)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/13/22 (3936 of 3942)
PASS: MLIR-Unit :: IR/./MLIRIRTests/37/142 (3937 of 3942)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/12/22 (3938 of 3942)
PASS: MLIR-Unit :: Pass/./MLIRPassTests/11/14 (3939 of 3942)
PASS: MLIR-Unit :: IR/./MLIRIRTests/38/142 (3940 of 3942)
PASS: MLIR :: mlir-reduce/dce-test.mlir (3941 of 3942)
command timed out: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2342.169189