ThreadDescriptorSize is only used by lsan to scan thread-specific data keys in
the thread control block.
On TLS Variant II architectures (i386/x86_64/s390), our dl_iterate_phdr
based approach can cover the region from the first byte of the static
TLS block (static TLS surplus) to the thread pointer.
We just need to extend the range to include the first few members of
struct pthread. offsetof(struct pthread, specific_used) satisfies
the requirement and has not changed since 2007-05-10. We don't need to
update ThreadDescriptorSize for each glibc version.