When determining the size of an object compute_objsize_r() assumes
that addresses derived from null pointers are not derefernceable
because null pointers themselves are not, without calling
targetm.addr_space.zero_address_valid() to see if that assumption
is supported for the pointer on the target.  The attached change
adds the missing call to avoid a spurious warning in Glibc that
uses named address spaces to implement TLS on x86_64.

Tested on x86_64-linux.

Martin