Only refer to base_debugging_flags in the target toolchain.
Only refer to base_debugging_flags in the target toolchain.

Previously this target exists for both the target and host toolchain. Since it
generates a file, when there is a separate host toolchain you will get
a warning about multiple targets generating the file.

BUG=567276

Review URL: https://codereview.chromium.org/1506913003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#363675}

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