[libc++] Fix broken configuration system-libcxxabi on Apple (#110920)

On Apple platforms, using system-libcxxabi as an ABI library wouldn't
work because we'd try to re-export symbols from libc++abi that the
system libc++abi.dylib might not have. Instead, only re-export those
symbols when we're using the in-tree libc++abi.

This does mean that libc++.dylib won't re-export any libc++abi symbols
when building against the system libc++abi, which could be fixed in
various ways. However, the best solution really depends on the intended
use case, so this patch doesn't try to solve that problem.

As a drive-by, also improve the diagnostic message when the user forgets
to set the LIBCXX_CXX_ABI_INCLUDE_PATHS variable, which would previously
lead to a confusing error.

Closes #104672

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