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Support for the wildcard is going away in GN, so this helps prepare for the transition away from GYP.

This cl adds dummy targets so that that logic for when the proper targets should be build can be kept in the platform-specific files.

Some of the conditions in the host gyp files needed to be restructured so that the dummy targets would be defined only when necessary.  The updated conditions caused a lot of lines to be re-indented, but the target definitions are the same (except for the new dummy targets).

BUG=

Review URL: https://codereview.chromium.org/896743004

Cr-Commit-Position: refs/heads/master@{#315150}