python: Fix m5's tick rounding mechanism

python: Fix m5's tick rounding mechanism

Partially reverts:
https://gem5-review.googlesource.com/c/public/gem5/+/29372 where the
`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-on: https://gem5-review.googlesource.com/c/public/gem5/+/29577
Reviewed-by: Tiago Mück <tiago.muck@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>