re PR go/89199 (libgo regression in implementation of CompareAndSwap functions resulting in intermittent testcase failures on ppc64le power9 after r268458)

	PR go/89199
    sync/atomic: use strong form of atomic_compare_exchange_n
    
    In the recent change to use atomic_compare_exchange_n I thought we
    could use the weak form, which can spuriously fail. But that is not
    how it is implemented in the gc library, and it is not what the rest
    of the library expects.
    
    Thanks to Lynn Boger for identifying the problem.
    
    Fixes https://gcc.gnu.org/PR89199
    
    Reviewed-on: https://go-review.googlesource.com/c/161359

From-SVN: r268591