RFR(S) 8067331: Zero: Atomic::xchg and Atomic::xchg_ptr need full memory barrier

[Severin Gehwolf]

Hi,

Could someone please review and sponsor the following change?

bug: https://bugs.openjdk.java.net/browse/JDK-8067331
webrev:
http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8067331/webrev.02/

The issue is that all atomic operations are expected to be full memory
barriers, but two implementations of the Zero variant JVM - Atomic::xchg
and Atomic::xchg_ptr - use GCC's built-in __sync_lock_test_and_set which
is an acquire barrier only. The fix adds full memory barriers manually
for those.

Thoughts?

Cheers,
Severin