[#557] Add system property 'os.version' to the list of eager substitutions

[olegchir]

Fixes #557.

Soft API change: new system property

oca-signed: Oleg Chirukhin
(http://www.oracle.com/technetwork/community/oca-486395.html)

[graalvmbot]

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[cstancu]

I think os.version should be a runtime computed value. By adding it to HOSTED_PROPERTIES you compute it at image build time, i.e., it is the os.version of the OS on which the image was built. While we can get away with it for os.arch and os.name for now since we don't have cross-compilation support the same doesn't apply for os.version.

[olegchir]

@cstancu ok, I'll investigate it further, thank you

[olpaw]

@olegchir I have a full implementation of os.version almost done that gets you the correct runtime value of os.version for linux and darwin. Stay tuned. It will land on master today or tomorrow.