[REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no longer reported in dr6

[Kyle Huey]

After resuming a ptracee with PTRACE_SINGLESTEP, in the following
ptrace stop retrieving the dr6 value for the tracee gets a value that
does not include DR_STEP (it is in fact always DR6_RESERVED). I
bisected this to the 13cb73490f475f8e7669f9288be0bcfa85399b1f merge. I
did not bisect further.

I don't see any handling to ever set DR_STEP in virtual_dr6, so I
think this code is just broken.

Sorry for not testing this when I was CCd on the original patch series :)

- Kyle