Don't run the t_cmsg_len testcase on 64-bit architectures

It always fails when trying to send through the sendit(9) private KPI in the
kernel due to a size mismatch between the msghdr and data being sent [*], which
suspiciously seems like it's related to sizeof pointers instead of scalars, or
something of that ilk

MFC after: 1 week
PR: 206543, 206544 [*]
Sponsored by: EMC / Isilon Storage Division