Updates for asm/ptrace/insn code

[Ben Dooks]

This is a re-send of the series that was setting up for the RDCYCLE
emulation in kernel space. I think these are still useful so am
re-sending this series with comments and fixups.

Even if we don't emulate RDCYCLE, we may still want to check for it
in the trap handler and possibly allow for it to be emulated later
if people feel that would be good (or at least some sort of warning
printed)

Thanks for the reviews. Hopefully this isn't too late for inclusion.