Fix rdr6() function on amd64

According to the AMD64 SysV ABI the first returned value is passed in RAX,
not in RDI. Actually RDI is used for the first argument passed to a
function.

So far this function was dead code, it will be used for a ptrace(2)
feature to support CPU watchpoints.

The rdr6() function reads state of the DR6 register and returns its value.

Sponsored by <The NetBSD Foundation>

(kamil)