[lldb] [Process/Utility] Fix DR offsets for FreeBSD

Fix Debug Register offsets to be specified relatively to UserArea
on FreeBSD/amd64 and FreeBSD/i386, and add them to UserArea on i386.
This fixes overlapping GPRs and DRs in gdb-remote protocol, making it
impossible to correctly get and set debug registers from the LLDB
client.

Differential Revision: https://reviews.llvm.org/D91254