pw_log: enable require_link_deps

Enables require_link_deps for the pw_log facade.

Migrates the upstream log backends to use the .impl targets.

Deprecates the pw_log_tokenized:log_backend target as
pw_log_tokenized can now be directly used thanks to
pw_build_LINK_DEPS.

Change-Id: I1a0ba11f4974f070ba7863a8007503f6cc628c7a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/45000
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Ewout van Bekkum <ewout@google.com>