Add compiler-rt build rules for LLVM sanitizers &co

Add build rules for LLVM in all the supported variations for NetBSD/amd64:

- asan
- xray
- tsan, dd
- libfuzzer
- msan
- safestack
- ubsan

Add compiler-rt build rules for LLVM sanitizers &co

Add build rules for LLVM in all the supported variations for NetBSD/amd64:

- asan
- xray
- tsan, dd
- libfuzzer
- msan
- safestack
- ubsan

These build rules compile functional sanitizers.

The build rules are still not hooked into the distribution build, as there
is pending work on backporting improvements for the llvm-8 branch needed
for netbsd-9 and newer.

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