Revert "build: Do not build under default toolchain"

This reverts commit e05b5458593aae7209ee3b9e8ea5096761327444.

Reason for revert: Broke downstream projects in unexpected ways

Original change's description:
> build: Do not build under default toolchain
>
> Moves some of the build targets generated by Pigweed's build to be under
> a non-default toolchain to prevent non-buildable targets from being
> instantiated by the default toolchain.
>
> Bug: b/226252176
> Requires: pigweed-internal:35048
> Change-Id: I073e952fa19250e0f2bedde4820f319bca1363b5
> Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/119310
> Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
> Reviewed-by: Anthony DiGirolamo <tonymd@google.com>
> Commit-Queue: Armando Montanez <amontanez@google.com>

TBR=amontanez@google.com,pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com,tonymd@google.com,auto-submit@pigweed.google.com.iam.gserviceaccount.com

Change-Id: I1b33d380a6f6fb9a986a14e7e2bdf06121b2782a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: b/226252176
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/119850
Reviewed-by: RJ Ascani <rjascani@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Armando Montanez <amontanez@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>