icedtea zero M68K atomic patch

[Xerxes Rånby]

Greetings!

Here is a patch for the M68K zero port to deal with missing atomic 
operations in M68K hardware, here implemented using a processor specific 
CAS instruction found on most 680X0 CPUs. It will priobably work on 
68020 68030 68040 and 68060.
Suggestions are welcome how to make improve it to work on all M68K cpus 
compatible with Linux/M68K like the ColdFire who lacks the CAS instruction.

I have included a small testprogram as well that can be build with g++ 
to test the zero atomic headerfile without compiling the whole hotspot.
xerxes at aranym-m68k:~/test$ g++ atomictest.cpp
xerxes at aranym-m68k:~/test$ ./a.out
start of test
testing Atomic::xchg
testing Atomic::xchg_ptr
a=C0FFEE b=CAFEBABE
Atomic::xchg() works!
c=EFE6A938 &b=EFE6A938 d=EFE6A93C &a=EFE6A93C
Atomic::xchg_ptr() works!

Have a great day!
Xerxes
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