[lldb] [Process/NetBSD] Fix reading FIP/FDP registers

Fix reading FIP/FDP registers to correctly return segment and offset
parts.  On amd64, this roughly matches the Linux behavior of splitting
the 64-bit FIP/FDP into two halves, and putting the higher 32 bits
into f*seg and lower into f*off.  Well, actually we use only 16 bits
of higher half but the CPUs do not seem to handle more than that anyway.

Differential Revision: https://reviews.llvm.org/D88681