This commit migrates from Azure Pipelines to Github Actions for CI for
cranelift. The CI configuration was relatively straightforward, and the
intention here is not to change what's actually being done on CI, just
change where it's being done. The previous CI configuration had build
targets for producing releases, but these weren't actually applicable
for cranelift itself (mostly just copied from wasmtime), so they've been
folded into the main "test everything" matrix which now includes
`--release` mode items.
See cranestation/wasmtime#474 for some more context as well in terms of
benefits of Github Actions vs Azure Pipelines.
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