Disable i386 on non-Linux platforms since it is unwanted and broken on Windows.
Please take a look.
I could be less conservative here and only disable i386 on Windows but I frankly don't think the risk of breaking other platforms is worth the benefit (nothing).
This is intended to fix https://bugs.chromium.org/p/chromium/issues/detail?id=957971#c5 I've verified that the "fuzzer" target is no longer possible to build on x86 (as opposed to possible but failing as it was before).
It's also worth noting that this breakage is in part caused by libFuzzer building with MSVC (__builtin_clzll isn't supported by MSVC but _BitScanReverse64 is supported, but not on i386) instead of indirectly with clang.
I'm also thinking i386 libFuzzer is broken on iOS (though I have no way of confirming this) and thus caused the greendragon bot failure: http://green.lab.llvm.org/green/job/clang-stage2-coverage-R/3958/consoleFull#console-section-35