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

[mgorny]

Summary

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.

lldb/source/Plugins/Process/Utility/RegisterContextFreeBSD_i386.cpp

struct GPR {
   uint32_t eip;
   uint32_t cs;
   uint32_t eflags;
   uint32_t esp;
   uint32_t ss;
   uint32_t gs;
 };
 
-struct dbreg {
+struct DBG {
   uint32_t dr[8]; /* debug registers */
                   /* Index 0-3: debug address registers */
                   /* Index 4-5: reserved */
                   /* Index 6: debug status */
                   /* Index 7: debug control */
 };
 
 using FPR_i386 = FXSAVE;
 
 struct UserArea {
   GPR gpr;
   FPR_i386 i387;
+  DBG dbg;
 };
 
 #define DR_SIZE sizeof(uint32_t)
-#define DR_OFFSET(reg_index) (LLVM_EXTENSION offsetof(dbreg, dr[reg_index]))
+#define DR_OFFSET(reg_index)                                                   \
+  (LLVM_EXTENSION offsetof(UserArea, dbg) +                                    \
+   LLVM_EXTENSION offsetof(DBG, dr[reg_index]))
 
 // Include RegisterInfos_i386 to declare our g_register_infos_i386 structure.
 #define DECLARE_REGISTER_INFOS_I386_STRUCT
 #include "RegisterInfos_i386.h"
 #undef DECLARE_REGISTER_INFOS_I386_STRUCT
 
 RegisterContextFreeBSD_i386::RegisterContextFreeBSD_i386(
     const ArchSpec &target_arch)

lldb/source/Plugins/Process/Utility/RegisterContextFreeBSD_x86_64.cpp

 };
 
 struct UserArea {
   GPR gpr;
   FPR fpr;
   DBG dbg;
 };
 
-#define DR_OFFSET(reg_index) (LLVM_EXTENSION offsetof(DBG, dr[reg_index]))
+#define DR_OFFSET(reg_index)                                                   \
+  (LLVM_EXTENSION offsetof(UserArea, dbg) +                                    \
+   LLVM_EXTENSION offsetof(DBG, dr[reg_index]))
 
 // Include RegisterInfos_x86_64 to declare our g_register_infos_x86_64
 // structure.
 #define DECLARE_REGISTER_INFOS_X86_64_STRUCT
 #include "RegisterInfos_x86_64.h"
 #undef DECLARE_REGISTER_INFOS_X86_64_STRUCT
 
 static std::vector<lldb_private::RegisterInfo> &GetSharedRegisterInfoVector() {