Draft patch adding LLVM sanitizers to the base

[Kamil Rytarowski <n54%gmx.com@localhost>]

I've prepared a draft patch adding LLVM sanitizers:

ASan, UBSan, TSan, Msan, SafeStack, libFuzzer, XRay and their variations

Please point out how to make it prettier and more aligned to the style
of other Makefiles.

http://netbsd.org/~kamil/llvm/llvm-sanitizers-patch1.txt

For now it just targets NetBSD/amd64, but keeps -m32 variation whenever
supported. Once the integration will be done, we will be able to extend
it with other ports.

I'm planning to import it once we will upgrade LLVM to 8.0 in base,
which will match the sanitizer ABI with imported compiler-rt.

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