Remove an x86-ism from RegisterInfoInterface

[labath]

Summary

While adding FPR support to x86 elf core files (D26300), we ended up adding a
very x86-specific function to the general RegisterInfoInterface class, which I
didn't catch in review. This removes that function. The only reason we needed
it was to find the offset of the FXSAVE area. This is the same as the offset of
the first register within that area, so we might as well use that.

[dvlahovski]

LGTM

lldb/trunk/source/Plugins/Process/Utility/RegisterContextLinux_i386.h

 #include "RegisterInfoInterface.h"
 
 class RegisterContextLinux_i386 : public lldb_private::RegisterInfoInterface {
 public:
   RegisterContextLinux_i386(const lldb_private::ArchSpec &target_arch);
 
   size_t GetGPRSize() const override;
 
-  size_t GetFXSAVEOffset() const override;
-
   const lldb_private::RegisterInfo *GetRegisterInfo() const override;
 
   uint32_t GetRegisterCount() const override;
 
   uint32_t GetUserRegisterCount() const override;
 
   const std::vector<lldb_private::RegisterInfo> *
   GetDynamicRegisterInfoP() const override;
 
 private:
   std::vector<lldb_private::RegisterInfo> d_register_infos;
 };
 
 #endif

lldb/trunk/source/Plugins/Process/Utility/RegisterContextLinux_i386.cpp

RegisterInfo orig_ax = {"orig_eax",
                           nullptr,
                           nullptr,
                           0};
   d_register_infos.push_back(orig_ax);
 }
 
 size_t RegisterContextLinux_i386::GetGPRSize() const { return sizeof(GPR); }
 
-size_t RegisterContextLinux_i386::GetFXSAVEOffset() const {
-  return (LLVM_EXTENSION offsetof(UserArea, i387));
-}
-
 const RegisterInfo *RegisterContextLinux_i386::GetRegisterInfo() const {
   switch (m_target_arch.GetMachine()) {
   case llvm::Triple::x86:
   case llvm::Triple::x86_64:
     return g_register_infos_i386;
   default:
     assert(false && "Unhandled target architecture.");
     return NULL;

lldb/trunk/source/Plugins/Process/Utility/RegisterContextLinux_x86_64.h

 #include "RegisterInfoInterface.h"
 
 class RegisterContextLinux_x86_64 : public lldb_private::RegisterInfoInterface {
 public:
   RegisterContextLinux_x86_64(const lldb_private::ArchSpec &target_arch);
 
   size_t GetGPRSize() const override;
 
-  size_t GetFXSAVEOffset() const override;
-
   const lldb_private::RegisterInfo *GetRegisterInfo() const override;
 
   uint32_t GetRegisterCount() const override;
 
   uint32_t GetUserRegisterCount() const override;
 
   const std::vector<lldb_private::RegisterInfo> *
   GetDynamicRegisterInfoP() const override;

lldb/trunk/source/Plugins/Process/Utility/RegisterContextLinux_x86_64.cpp

RegisterInfo orig_ax = {"orig_rax",
                           nullptr,
                           nullptr,
                           0};
   d_register_infos.push_back(orig_ax);
 }
 
 size_t RegisterContextLinux_x86_64::GetGPRSize() const { return sizeof(GPR); }
 
-size_t RegisterContextLinux_x86_64::GetFXSAVEOffset() const {
-  return (LLVM_EXTENSION offsetof(UserArea, fpr) +
-          LLVM_EXTENSION offsetof(FPR, xstate));
-}
-
 const std::vector<lldb_private::RegisterInfo> *
 RegisterContextLinux_x86_64::GetDynamicRegisterInfoP() const {
   return &d_register_infos;
 }
 
 const RegisterInfo *RegisterContextLinux_x86_64::GetRegisterInfo() const {
   return m_register_info_p;
 }
 
 uint32_t RegisterContextLinux_x86_64::GetRegisterCount() const {
   return m_register_info_count;
 }
 
 uint32_t RegisterContextLinux_x86_64::GetUserRegisterCount() const {
   return m_user_register_count;
 }

lldb/trunk/source/Plugins/Process/Utility/RegisterContextPOSIX_x86.cpp

size_t RegisterContextPOSIX_x86::GetRegisterCount() {
   return num_registers;
 }
 
 size_t RegisterContextPOSIX_x86::GetGPRSize() {
   return m_register_info_ap->GetGPRSize();
 }
 
 size_t RegisterContextPOSIX_x86::GetFXSAVEOffset() {
-  return m_register_info_ap->GetFXSAVEOffset();
+  return GetRegisterInfo()[m_reg_info.first_fpr].byte_offset;
 }
 
 const RegisterInfo *RegisterContextPOSIX_x86::GetRegisterInfo() {
   // Commonly, this method is overridden and g_register_infos is copied and
   // specialized.
   // So, use GetRegisterInfo() rather than g_register_infos in this scope.
   return m_register_info_ap->GetRegisterInfo();
 }

lldb/trunk/source/Plugins/Process/Utility/RegisterInfoInterface.h

 class RegisterInfoInterface {
 public:
   RegisterInfoInterface(const lldb_private::ArchSpec &target_arch)
       : m_target_arch(target_arch) {}
   virtual ~RegisterInfoInterface() {}
 
   virtual size_t GetGPRSize() const = 0;
 
-  virtual size_t GetFXSAVEOffset() const { return 0; }
-
   virtual const lldb_private::RegisterInfo *GetRegisterInfo() const = 0;
 
   // Returns the number of registers including the user registers and the
   // lldb internal registers also
   virtual uint32_t GetRegisterCount() const = 0;
 
   // Returns the number of the user registers (excluding the registers
   // kept for lldb internal use only). Subclasses should override it if