Remove the arg0 field from struct amd64_frame. Its existence was a bug,
since on amd64 the first argument to a function is generally not on the
stack.

Revert an old DTrace bug fix to some code that assumed that
sizeof(struct amd64_frame) == 16.

Reviewed by: jhb, kib
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D3255