systemc: Stop assuming picoseconds as the time resolution.

Also adjust some code to avoid floating point rounding problems and
integer overflow issues.

Change-Id: Ib4b9c4cf4af00333951db5ce07819556141aa5da
Reviewed-on: https://gem5-review.googlesource.com/c/12614
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
1 file changed