[Mips] Update UNW_TDEP_LAST_REG to take in account the UNW_MIPS_PC
register index. Use the UNW_MIPS_PC for UNW_TDEP_IP macro. The $31
register is not a program counter on MIPS. It is a value of a return
address for jal calls.
[Mips] Update MIPS registry access constants in the _UPT_reg_offset array.
Use a magic value '64' to access a program counter register. This constant
is defined in the arch/mips/include/uapi/asm/ptrace.h file.