build: prevent accidental builds with clang in builder

[benesch]

The default toolchain in the builder image is crashy, but plausibly
useful for msan builds. Teach the Makefile to prohibit use of the crashy
toolchain unless an msan build is explicitly requested.

Fix #16071.

Release note: None

[blathers-crl]

Thank you for contributing to CockroachDB. Please ensure you have followed the guidelines for creating a PR.

My owl senses detect your PR is good for review. Please keep an eye out for any test failures in CI.

I have added a few people who may be able to assist in reviewing:

• @kenliu (commented on build: use a gcc-based toolchain by default (on linux and in builder) #16071)
• @tbg (author of build: use a gcc-based toolchain by default (on linux and in builder) #16071)
• @petermattis (commented on build: use a gcc-based toolchain by default (on linux and in builder) #16071)
• @irfansharif (commented on build: use a gcc-based toolchain by default (on linux and in builder) #16071)

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[cockroach-teamcity]

This change is 

[blathers-crl]

❌ The GitHub CI (Cockroach) build has failed on 1d0680ab.

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[benesch]

Well, this has exposed that CI runs all tests with the crashy toolchain. Surprising that works. Anyway, I'm not planning to debug this further, but hopefully it's useful information for whoever winds up working on this.

[petermattis]

Well, this has exposed that CI runs all tests with the crashy toolchain. Surprising that works. Anyway, I'm not planning to debug this further, but hopefully it's useful information for whoever winds up working on this.

Thanks for the attempt.

Note that I don't think the clang tool chain is always crashy. My understanding is that crashes only occur when using the clang tool chain and cgosymbolizer. The latter is only used when CPU profiling is enabled. Heap profiles (which are always enabled) are unaffected because the associated stacks are always rooted in Go.