Also use the name "gen_snapshot" when building for Android targets
instead of appending a target architecture to the filename.
This allows flutter_tools to find the universal gen_snapshot binary when
using a locally built engine. Previously the tools would search
directories like "clang_x64" and find build outputs that may not match
the host architecture.
Moving the universal gen_snapshot binary to a separate directory avoids
conflicts with Dart's gen_snapshot binary, which is placed at the root
of the target output directory.
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