RFR(M): 8194258: PPC64 safepoint mechanism: Fix initialization on AIX and support SIGTRAP

[Doerr, Martin]

Hi,

unfortunately, the safepoint mechanism initialization on AIX is not completely correct, yet. The VM crashes when using -XX:-OptimizePollingPageLocation or when allocation at desired addresses failed.
Only pages allocated by mmap can be read protected.

In addition, SIGTRAP-based polling performs slightly better on PPC64 and avoids the need for allocating a protected page.

I have uploaded a webrev:
http://cr.openjdk.java.net/~mdoerr/8194258_PPC64_poll/webrev.00/

We use trap instructions if UseSIGTRAP is enabled (PPC64 specific -XX flag) and fall back to loads from the polling pages otherwise.

Please review.

I will also need a sponsor because I'm changing the shared file safepointMechanism.cpp. It's not a functional change for other platforms.

Best regards,
Martin