RFR: 8255065: Zero: accessor_entry misses the IRIW case

[Aleksey Shipilev]

While doing a change in related area, I noticed there is no IRIW handling block in `ZeroInterpreter::accessor_entry`
when reading volatile fields. This probably breaks PPC64 Zero.

There is a block in `bytecodeInterpreter.cpp` for [common field
access](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/interpreter/zero/bytecodeInterpreter.cpp#L1899-L1901):

          if (cache->is_volatile()) {
            if (support_IRIW_for_not_multiple_copy_atomic_cpu) {
              OrderAccess::fence();
            }

Attention @TheRealMDoerr ;)

Testing:
 - [x] Linux x86_64 zero fastdebug build (includes jmod generation with Zero)

-------------

Commit messages:
 - 8255065: Zero: accessor_entry misses the IRIW case

Changes: https://git.openjdk.java.net/jdk/pull/766/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=766&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255065
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/766.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/766/head:pull/766

PR: https://git.openjdk.java.net/jdk/pull/766