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