[lldb] [Process/NetBSD] Set xs_xstate_bv correctly when setting regs

Ensure that xs_xstate_bv is set correctly before calling
WriteRegisterSet().  The bit can be clear if the relevant registers
were at their initial state when they were read, and it needs to be set
in order to apply changes from the XState structure.

Differential Revision: https://reviews.llvm.org/D90105