dtrace: Fix up %rip for invop probes on x86

When a breakpoint exception is raised, the saved value of %rip points to
the instruction following the breakpoint. However, when fetching the
value of %rip using regs[], it's more natural to provide the address of
the breakpoint itself, so modify the kinst and fbt providers accordingly.

Reported by: khng
Reviewed by: christos, khng
MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D37218