RFR (S): JDK-8066599: Add methods to check VM mode to c.o.j.t.Platform

[Filipp Zhinkin]

Hi all,

please review a small change that adds VM mode checking methods to
jdk.test.lib.Platform.
I didn't touch Platform class from hotspot/test/testlibrary, because
it is already deprecated and at some point of time all hs tests will
be updated to use j.t.l.Platform instead.
Update of hs tests that use own logic to check VM mode is tracked by
JDK-8145848 [1].

Webrev: http://cr.openjdk.java.net/~fzhinkin/8066599/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8066599
Testing: on local machine with different VM modes.

Regards,
Filipp.

[1] https://bugs.openjdk.java.net/browse/JDK-8145848