RFR (M): 8187977: Generalize Atomic::xchg to use templates

[Erik Österlund]

Hi all,

The time has come to generalize more atomics.

I have modelled Atomic::xchg to systematically do what Atomic::cmpxchg
did but for xchg.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8187977

Webrev:
http://cr.openjdk.java.net/~eosterlund/8187977/webrev.00/

Testing: mach5 hs-tier3 and JPRT

Thanks,
/Erik