Cirrus-CI: switch to Clang/LLVM 12

[emaste]

Summary

@dim is preparing to import Clang/LLVM 12 into the base system as the system compiler / toolchain. Apply the same change to the Cirrus-CI config.

[emaste]

Running now at https://cirrus-ci.com/task/4987267849650176

[emaste]

Build + boot smoke test completed successfully.

[imp]

Once upon a time, you also needed to suppress bootstrapping the toolchain... Does CROSS_TOOLCHAIN do that automatically these days?

[emaste]

In D30109#675892, @imp wrote:

Once upon a time, you also needed to suppress bootstrapping the toolchain... Does CROSS_TOOLCHAIN do that automatically these days?

I believe so, but will double check.

[emaste]

I believe so, but will double check.

Looking at the Cirrus-CI log for the run above, I do not see Clang being built (bootstrap or for target).

[markj]

In D30109#675894, @emaste wrote:

In D30109#675892, @imp wrote:

Once upon a time, you also needed to suppress bootstrapping the toolchain... Does CROSS_TOOLCHAIN do that automatically these days?

I believe so, but will double check.

Isn't that what WITHOUT_TOOLCHAIN= accomplishes?

[emaste]

Isn't that what WITHOUT_TOOLCHAIN= accomplishes?

That should avoiding the toolchain that would be installed, but (with only that set) would still build the bootstrap toolchain.