ARC: Make way for pt_regs != user_regs_struct

These have been register compatible so far. However ARCv2 mandates
different pt_regs layout (due to h/w auto save). To keep pt_regs same
for both, we start by removing the assumption - used mainly for block
copies between the 2 structs in signal handling and ptrace

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>