dtrace: 64-bits registers support

The registers in ilumos and FreeBSD have a different number.
In the illumos, last 32-bits register defined is SS an in FreeBSD is GS.
This off-by-one caused the uregs array to returns the wrong 64-bits register
on amd64.

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20363