RFR (S): 8033380: Experimental VM flag to enforce access atomicity

[Aleksey Shipilev]

Hi,

I understand we are still closed for integration?

Please review this small feature meanwhile:
  https://bugs.openjdk.java.net/browse/JDK-8033380
  http://cr.openjdk.java.net/~shade/8033380/webrev.02/

TL;DR: JMM 9 may need to extend the access atomicity over longs and
doubles. Luckily, our logic in emitting the relevant access-atomic
instructions is decoupled from memory barriers logic, and so we can
"just" unconditionally go through needs_atomic_access where appropriate.

Testing:
 - full cycle JPRT
 - targeted microbenchmarks on x86/ARM/PPC

Thanks,
-Aleksey