[OR1K] Add a dedicated PC register to register state.

Before this commit, R9, the link register, was used as PC register.
However, a stack frame may have R9 not set to PC on entry, either
because it uses a custom calling convention, or, more likely,
because this is a signal or exception stack frame. Using R9 as
PC register made it impossible to unwind such frames.

All other architectures similarly use a dedicated PC register.

llvm-svn: 332512