RegisterInfoPOSIX_arm64 remove unused bytes from g/G packet

This came up while putting together our new strategy to create g/G packets
in compliance with GDB RSP protocol where register offsets are calculated in
increasing order of register numbers without any unused spacing.

RegisterInfoPOSIX_arm64::GPR size was being calculated after alignment
correction to 8 bytes which meant there was a 4 bytes unused space between
last gpr (cpsr) and first vector register V. We have put LLVM_PACKED_START
decorator on RegisterInfoPOSIX_arm64::GPR to make sure single byte
alignment is enforced. Moreover we are now doing to use arm64 user_pt_regs
struct defined in ptrace.h for accessing ptrace user registers.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D92063

lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_arm64.cpp

@@ -936,9 +936,9 @@ Status NativeRegisterContextLinux_arm64::ReadGPR() {
 
   struct iovec ioVec;
   ioVec.iov_base = GetGPRBuffer();
-  ioVec.iov_len = GetGPRSize();
+  ioVec.iov_len = GetGPRBufferSize();
 
-  error = ReadRegisterSet(&ioVec, GetGPRSize(), NT_PRSTATUS);
+  error = ReadRegisterSet(&ioVec, GetGPRBufferSize(), NT_PRSTATUS);
 
   if (error.Success())
     m_gpr_is_valid = true;
@@ -953,11 +953,11 @@ Status NativeRegisterContextLinux_arm64::WriteGPR() {
 
   struct iovec ioVec;
   ioVec.iov_base = GetGPRBuffer();
-  ioVec.iov_len = GetGPRSize();
+  ioVec.iov_len = GetGPRBufferSize();
 
   m_gpr_is_valid = false;
 
-  return WriteRegisterSet(&ioVec, GetGPRSize(), NT_PRSTATUS);
+  return WriteRegisterSet(&ioVec, GetGPRBufferSize(), NT_PRSTATUS);
 }
 
 Status NativeRegisterContextLinux_arm64::ReadFPR() {

lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_arm64.h

@@ -95,6 +95,10 @@ class NativeRegisterContextLinux_arm64 : public NativeRegisterContextLinux {
 
   void *GetGPRBuffer() override { return &m_gpr_arm64; }
 
+  // GetGPRBufferSize returns sizeof arm64 GPR ptrace buffer, it is different
+  // from GetGPRSize which returns sizeof RegisterInfoPOSIX_arm64::GPR.
+  size_t GetGPRBufferSize() { return sizeof(m_gpr_arm64); }
+
   void *GetFPRBuffer() override { return &m_fpr; }
 
   size_t GetFPRSize() override { return sizeof(m_fpr); }
@@ -106,7 +110,7 @@ class NativeRegisterContextLinux_arm64 : public NativeRegisterContextLinux {
 
   bool m_sve_header_is_valid;
 
-  RegisterInfoPOSIX_arm64::GPR m_gpr_arm64; // 64-bit general purpose registers.
+  struct user_pt_regs m_gpr_arm64; // 64-bit general purpose registers.
 
   RegisterInfoPOSIX_arm64::FPU
       m_fpr; // floating-point registers including extended register sets.

lldb/source/Plugins/Process/Utility/RegisterInfoPOSIX_arm64.h

@@ -29,6 +29,7 @@ class RegisterInfoPOSIX_arm64
   };
 
   // based on RegisterContextDarwin_arm64.h
+  LLVM_PACKED_START
   struct GPR {
     uint64_t x[29]; // x0-x28
     uint64_t fp;    // x29
@@ -37,6 +38,7 @@ class RegisterInfoPOSIX_arm64
     uint64_t pc;    // pc
     uint32_t cpsr;  // cpsr
   };
+  LLVM_PACKED_END
 
   // based on RegisterContextDarwin_arm64.h
   struct VReg {