```python: Fix m5's tick rounding mechanism

Partially reverts:
`math.ceil` function was used to fix an issue where 0.5 was rounded to
zero in python3. This has the side effect of giving incorrect clock
frequences due to rounding errors. To demonstrate:

```
>>> import math
>>> 1e-9*1000000000000
1000.0000000000001
>>> int(math.ceil(1e-9*1000000000000))
1001
```

The solution to this problem is to use Python's `decimal` module to
round values.

Issue-on: https://gem5.atlassian.net/browse/GEM5-616
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Change-Id: Ic76736ccb4b6b8c037103a34493aff7d9731d314
Reviewed-by: Tiago Mück <tiago.muck@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
```
```diff --git a/src/python/m5/ticks.py b/src/python/m5/ticks.py
index 047116a..cdba1e8 100644
--- a/src/python/m5/ticks.py
+++ b/src/python/m5/ticks.py
```
```@@ -25,7 +25,7 @@
# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

from __future__ import print_function
-from math import ceil
+import decimal
import six
if six.PY3:
long = int
@@ -74,7 +74,8 @@
# convert the value from time to ticks
value *= _m5.core.getClockFrequency()

-    int_value = int(ceil(value))
+    int_value = int(
+            decimal.Decimal(value).to_integral_value( decimal.ROUND_HALF_UP))
err = (value - int_value) / value
if err > frequency_tolerance:
warn("rounding error > tolerance\n    %f rounded to %d", value,
```