RFR (S) 8009575 (2nd) - Reduce Symbol::_refcount from 4 bytes to 2 bytes

[Ioi Lam]

Please review my 2nd trial for this bug fix. It's much simplified thanks
to Coleen's suggestion.

http://cr.openjdk.java.net/~iklam/8009575/symbol_refcount_002/

Bug: Reduce Symbol::_refcount from 4 bytes to 2 bytes

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8009575
     https://jbs.oracle.com/bugs/browse/JDK-8009575

Summary of fix:

     As noted in the bug report, the main problem is _refcount needs to be
     atomically incremented, but the smallest unit that Atomic::incr() had
     supported until now was 32-bit int.

     With Coleen's help, I have created a platform-independent version
     of Atomic::incr(short*) that's based on Atomic::add(int*, n).

     Essentially, Atomic::incr(short*) can be implemented as
     Atomic::add(int*, 0x10000), as long as the short occupies the most
     significant 16 bits of the int. I added the macro ATOMIC_SHORT_PAIR
     to ensure the proper alignment.

     Also Symbol::size(int) is fixed to calculate the correct space needed
     for a Symbol. (Same as in the previos webrev --
     see 
http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2013-March/006385.html)

Tests:

     JPRT
     UTE:
        - vm.tmtools.testlist nsk.sajdi.testlist vm.runtime.testlist
          vm.quick.testlist vm.parallel_class_loading.testlist
        - Serviceability Agent tested are included in these lists.
     MedRec

Thanks
- Ioi