[nsan] Add nsan_preinit.cpp and make it static library only

#94322 defines .preinit_array to initialize nsan early.
DT_PREINIT_ARRAY can only be used with the main executable. GNU ld would
complain when a DSO has .preinit_array. Therefore,
nsan_preinit.cpp cannot be linked into `libclang_rt.nsan.so` (#98415).

Working with @alexander-shaposhnikov, we noticed that `Nsan-x86_64-Test
--gtest_output=json` without `.preinit_array` will sigsegv. This is
because googletest with the JSON output calls `localtime_r` , which
calls `free(0)` and fails when `REAL(free)` remains uninitialized
(nullptr). This is benign with the default output because malloc/free
are all paired and `REAL(free)(ptr)` is not called.

To fix the unittest failure, `__nsan_init` needs to be called early
(.preinit_array).
`asan/tests/CMakeLists.txt:ASAN_UNITTEST_INSTRUMENTED_LINK_FLAGS` ues
`-fsanitize=address` to ensure `asan_preinit.cpp.o` is linked into the
unittest executable. Port the approach and remove
`NSAN_TEST_RUNTIME_OBJECTS`.

Fix #98523

Pull Request: https://github.com/llvm/llvm-project/pull/98564

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