Rework the LLVM compiler_rt build rules

Register external/bsd/compiler_rt as a _SUBDIR. This will allow to handle
properly includes specific for compiler_rt.

Centralize build_install target for external/bsd/compiler_rt.

With the above changes MKOBJDIR is now respected properly for me.

Do not install LLVM sanitizers into TOOLDIR for !HAVE_LLVM.

branches: 1.1.2;
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.

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.