Make common signature system property definitions optional

This change allows ARC to define ARC-specific system properties in
Android build without changing these property definitions for other
Android builds. Please see go/arc-sigprop-changes and
go/arc-android-sigprop-sync for additional details.

Bug: 195609932
Test: built bertha_x86_64 with forward declarations
Change-Id: I22bd9d60c2491506fe5c633dbbb9e7516f529b35