[compiler-rt] Fix -shared-libsan usage with internal symbolizer (#227717)

Summary:
https://github.com/llvm/llvm-project/pull/226551 exposed a preexisting
issue when using `-shared-libsan` nad the internal symbolizer builds.
The symbolizer will try to look up the symbol via `RTLD_NEXT`, which
goes in load order. If the sanitizer library is loaded *after* `libc.so`
then this `dlsym` call will miss it.

The solution is to have a fallback that checks using `RTLD_DEFAULT`.
This
should allow us to find the symbol in these exceptional cases. I don't
expect much fallout as this covers a case that currently returns `NULL`.
Loading it in means it could grab a reference ahead of the runtime that
was intended to be intercepted, but for this use-case I don't think this
will apply.

GitOrigin-RevId: d47315ba62d6b572a2e6aa0dd7f74a3cc128a569