Clock: Add a Cycles wrapper class and use where applicable

This patch addresses the comments and feedback on the preceding patch
that reworks the clocks and now more clearly shows where cycles
(relative cycle counts) are used to express time.

Instead of bumping the existing patch I chose to make this a separate
patch, merely to try and focus the discussion around a smaller set of
changes. The two patches will be pushed together though.

This changes done as part of this patch are mostly following directly
from the introduction of the wrapper class, and change enough code to
make things compile and run again. There are definitely more places
where int/uint/Tick is still used to represent cycles, and it will
take some time to chase them all down. Similarly, a lot of parameters
should be changed from Param.Tick and Param.Unsigned to
Param.Cycles.

In addition, the use of curTick is questionable as there should not be
an absolute cycle. Potential solutions can be built on top of this
patch. There is a similar situation in the o3 CPU where
lastRunningCycle is currently counting in Cycles, and is still an
absolute time. More discussion to be had in other words.

An additional change that would be appropriate in the future is to
perform a similar wrapping of Tick and probably also introduce a
Ticks class along with suitable operators for all these classes.

src/python/m5/params.py

@@ -463,6 +463,8 @@ class CheckedInt(NumericParamValue):
         # most derived types require this, so we just do it here once
         code('%import "stdint.i"')
         code('%import "base/types.hh"')
+        # ignore the case operator for Cycles
+        code('%ignore *::operator uint64_t() const;')
 
     def getValue(self):
         return long(self.value)
@@ -480,6 +482,7 @@ class Int64(CheckedInt):    cxx_type =  'int64_t'; size = 64; unsigned = False
 class UInt64(CheckedInt):   cxx_type = 'uint64_t'; size = 64; unsigned = True
 
 class Counter(CheckedInt):  cxx_type = 'Counter';  size = 64; unsigned = True
+class Cycles(CheckedInt): cxx_type = 'Cycles'; size = 64; unsigned = True
 class Tick(CheckedInt):     cxx_type = 'Tick';     size = 64; unsigned = True
 class TcpPort(CheckedInt):  cxx_type = 'uint16_t'; size = 16; unsigned = True
 class UdpPort(CheckedInt):  cxx_type = 'uint16_t'; size = 16; unsigned = True