LegalizeDAG: allow libcalls for max/min atomic operations

ARM processors without ldrex/strex need to be able to make libcalls for all
atomic operations, including the newer min/max versions.

The alternative would probably be expanding these operations in terms of
cmpxchg (as x86 does always), but in the configurations where this matters
code-size tends to be paramount so the libcall is more desirable.

llvm-svn: 193398