[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