default to -march=i586 on i386

llvm 9 and later take more care not to inline cmpxchg8b/cx8 with
-march=i486 instead calling __atomic_* functions.  This in turn breaks
the build of a large number of ports.

To avoid having to add a lock or conditionally building individual ports
with -march=i586 change the default so 64 bit atomics will always be
available.

ok patrick@ sthen@ deraadt@

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