PR libstdc++/3584
* config/cpu/arm/atomicity.h (__exchange_and_add): Don't try to use
ASM sequences that don't give us atomic addition operations.  Instead
just add a comment explaining why it doesn't work.
(__atomic_add): Just use __exchange_and_add.
(__test_and_set, __always_swap): Delete.

From-SVN: r57778