Bring back the logic to disable debugging symbols for 32
bit native compilers on Windows (symbol_level = 0), or
only do minimal debugging symbols useful for e.g. stack
traces (symbol_level = 1).

The corresponding gyp logic got removed in commit c5282c34

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Change-Id: Ibb7d43814992a00f5413d024f9c362d41dc5a3a5
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>