[nsan] Add shared runtime

so that `clang -fsanitize=numerical -shared-libsan` will use
`libclang_rt.nsan.so` on Linux.

Shared runtime is preferred for some platforms (Android, Apple, Fuchsia;
though they are not supported yet) and helps plugin use cases (#98302).

* Update `ninja nsan` to build `libclang_rt.nsan.so`
* Fix `nsan.syms.extra`: `nsan_*` is unneeded. Add `__ubsan_*` so that
  `-fsanitize=numerical,undefined -shared-libsan` works.
* Move allocation functions to `nsan_malloc_linux.cpp`. While Apple
  platforms aren't supported yet, this separation makes it easier to add
  Apple support.
* Delete interceptors for very obsoleted pvalloc/valloc but retain
  memalign.
* Replace `HandleEarlyAlloc` with `DlsymAlloc`.

Pull Request: https://github.com/llvm/llvm-project/pull/98415

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