fix x32 msghdr struct by removing x32 bits/socket.h

fix x32 msghdr struct by removing x32 bits/socket.h

being that it contains pointers and (from the kernel perspective,
which is wrong) size_t members, x32 uses the 32-bit version of the
structure, not a half-32-bit, half-64-bit layout like we had here. the
x86_64 definition was inadvertently copied when x32 was first added.

unlike errors in the opposite direction (missing padding), this error
was not easily detected breakage, because the layout of the commonly
used initial subset of members still matched. breakage could only be
observed in the presence of control messages or flags.