[libcxxabi] Fix printf formats in a test.

This is the libcxxabi counterpart of D89545, and would have been part
of that patch if I'd spotted it soon enough (oops). One test in
libcxxabi is using the `%lu` printf format to refer to `size_t`, which
should be `%zu`.

Reviewed By: ldionne, #libc_abi

Differential Revision: https://reviews.llvm.org/D89547