Because FreeBSD no longer supports the 80386 cpu all code targeting
FreeBSD/i386 necessarily runs on i486 or higher so the compiler
built-ins can be used by default inside libstdc++ and in C++ headers.
This allows newly compiled C++ code to inline some atomic operations.
Old binaries continue to use libstdc++ functions.
PR: 148926
Tested by: Yuri Karaban <tech askold net>
Reviewed by: kan
Approved by: kib (mentor)
MFC after: 2 weeks
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