Allow passing labels for builtin_sysroot instead of string

[shmuelamit]

Description of the feature request:

For some non-obvious reason create_cc_toolchain_config_info gets the field builtin_sysroot as a string instead of as a label. This causes some major projects to use a hack in which they depend on the hard-coded path external/{target name} instead of a target label which imo will be a better approach. For example:

• mono/skia
• google/pigweed
• aspect-build/gcc-toolchain
  And more... I think bazel should provide a suitable alternative which does not use these kinds of hacks. Such a solution may exist out there, yet I have not found one.

Which category does this issue belong to?

Rules API

What underlying problem are you trying to solve with this feature?

The fact that users of toolchains need to hard-code sysroot paths instead of using the natural approach which is labels.

Which operating system are you running Bazel on?

Rocky Linux

What is the output of bazel info release?

release 6.4.0

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

No response

Have you found anything relevant by searching the web?

Stack overflow which shows how to use sysroot path
https://stackoverflow.com/questions/62451307/specify-sysroot-for-bazel-toolchain

My Slack thread
https://bazelbuild.slack.com/archives/CA31HN1T3/p1706711506150469

Any other information, logs, or outputs that you want to share?

No response