Restore host and nonhost toolchain features (reversing #7407)

[chandlerc]

Description of the problem / feature request:

Currently it is necessary to pass --incompatible_dont_enable_host_nonhost_crosstool_features=false to get host and nonhost features back in the C++ toolchain.

In #7407 these were suggested to be removed because there may not be a separate host and target platform or toolchain. That is true, but features in C++ toolchains control much more than just which platform is used or which toolchain is used. There are features that make sense to enable in the host build but not in the target build even when the platforms are identical.

Some easy examples are the LLVM sanitizers. These are debugging aids but can come with high runtime overhead. It is often undesirable to subject host tools to these overheads, and instead they should only be enabled for target builds.

Another example is disabling debug information in host builds: #13308

Not sure anything else is needed here, but if there are questions about how this might be used, please let me know.

[hlopko]

Just sharing the workaround: it seems ctx.genfiles_dir.path.find("-exec-") can be used to detect exec configuration.

[github-actions]

Thank you for contributing to the Bazel repository! This issue has been marked as stale since it has not had any activity in the last 1+ years. It will be closed in the next 14 days unless any other activity occurs or one of the following labels is added: "not stale", "awaiting-bazeler". Please reach out to the triage team (@bazelbuild/triage) if you think this issue is still relevant or you are interested in getting the issue resolved.

[github-actions]

This issue has been automatically closed due to inactivity. If you're still interested in pursuing this, please reach out to the triage team (@bazelbuild/triage). Thanks!

[chandlerc]

FWIW, I would love an actual update here rather than just closing this as stale.