RFR(s): 8198267: Allow GCId::current_raw() calls from non-NamedThreads

[Kim Barrett]

> On Feb 20, 2018, at 1:32 AM, Kim Barrett <kim.barrett at oracle.com> wrote:
> The inability to call GCId::current_raw from non-NamedThreads is what
> led me to jump through some hoops in the gtests for OopStorage,
> sometimes needing to transition a thread from native to VM.

Looking at the test code more carefully, I think the transition from native to VM was to
deal with some assert_in_safepoints in some places in OopStorage’s implementation.

The GCId issue seems to have been dealt with by explicitly passing GCId::undefined() to
the AbstractGangTask constructor.  The proposed default behavior would work for the
oopstorage gtest, but the test still seems like it needs updating.