sync/atomic: disable hammer pointer tests on wrong size system
sync/atomic: disable hammer pointer tests on wrong size system

hammerCompareAndSwapPointer64 was only passing on
little-endian systems.  hammerCompareAndSwapPointer32 was
writing 8 bytes to a uint32 value on the heap.

R=rsc, dvyukov
CC=golang-dev
http://codereview.appspot.com/5654065

d0f/go-zh,Triskite/willstone-goclone,d0f/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,webfd/go-zh,d0f/go-zh,Triskite/willstone-goclone,mhennings/marcohennings-go,scirelli/scirelli-go,bryanxu/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,webfd/go-zh,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,webfd/go-zh,scirelli/scirelli-go,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,Triskite/willstone-goclone,mhennings/marcohennings-go,scirelli/scirelli-go,mhennings/marcohennings-go,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,scirelli/scirelli-go,Triskite/willstone-goclone,webfd/go-zh,webfd/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang