i686-apple-darwin chooses a pre-pentiumpro cpu by default

[gnzlbg]

The first 32-bit x86 apple machine used the "yonah" CPU, but we (and clang) set a generic i686 CPU instead that has no MMX, no SSE, etc.

This is also clang bug: https://bugs.llvm.org/show_bug.cgi?id=38594

We should wait and see how clang resolves this bug before doing anything.

[gnzlbg]

This appears to be fixed now: https://rust.godbolt.org/z/UqjUHQ the default CPU is "yonah".