build: Add `pw_emboss_build_dir_to_root` arg

This change introduces the `pw_emboss_build_dir_to_root` arg, allowing
the `root_build_dir` used by the `emboss_cc_library` build rule to be
overridden when passing in rebased source file to `embossc`.

This helps ensure the correct placement of generated emboss header files
when the build/output directory is a symlink pointing outside of `//`,
which happens for cog-based fuchsia workspaces.

See the comment added to `//third_party/emboss/build_defs.gni` and
https://fxrev.dev/1416994 for additional context.

Bug: 460495607
Change-Id: I4c3c05ea9da64a0f5499176d6af080d6831f1255
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/342332
Reviewed-by: Ben Lawson <benlawson@google.com>
Docs-Not-Needed: Ben Lawson <benlawson@google.com>
Pigweed-Auto-Submit: Darren Chan <chandarren@google.com>
Copybara-Verified: Copybara Prod <copybara-worker-blackhole@google.com>
Commit-Queue: Darren Chan <chandarren@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>