對 Fuchsia 建構目錄執行 gn gen。
Execute a `gn gen` against the Fuchsia build
If successful, execute a `gn gen` against the Fuchsia build.
In abstract, if FUCHSIA_BUILD_DIR is out/x64, this is simply:
gn gen out/x64
It is useful if one has by some mechanism deleted the ninja artifacts, but
not the args.gn, e.g. if one CTRL+C's a regen step (gn bug).