Disable ASan exceptions on NetBSD

This is a workarond for the fallout from D42644:
[asan] Intercept std::rethrow_exception indirectly.

Reported problem on NetBSD/amd64:

$ sh ./projects/compiler-rt/test/sanitizer_common/asan-i386-NetBSD/NetBSD/Output/ttyent.cc.script
/usr/lib/i386/libgcc.a(unwind-dw2.o): In function `_Unwind_RaiseException':
unwind-dw2.c:(.text+0x1b41): multiple definition of `_Unwind_RaiseException'
/public/llvm-build/lib/clang/7.0.0/lib/netbsd/libclang_rt.asan-i386.a(asan_interceptors.cc.o):/public/llvm/projects/compiler-rt/lib/asan/asan_interceptors.cc:337: first defined here
clang-7.0: error: linker command failed with exit code 1 (use -v to see invocation)

llvm-svn: 326216