Fix to step instruction due to P10 prefix instruction.

In AIX, power 10 instructions like paddi occupy 8 bytes, while the other instructions
4 bytes of space. Due to this when we do a stepi on paddi instruction we get a SIGILL interrupt. Hence, we
need to check during stepi if we are able to step 8 bytes during this instruction execution and is the
breakpoint to this instruction set correctly in both 32- and 64-bit mode.

This patch is a fix to the same.