[lldb] [Process/FreeBSDRemote] Optimize regset pointer logic

Create a helper GetOffsetRegSetData() method to get pointer
to the regset data accounting for the necessary offset.  Establish
the offsets in the constructor and store them in the structure.  This
avoids having to add new Get*Offset() methods and combines some common
code.

Differential Revision: https://reviews.llvm.org/D91411