Revert switch to bc to do fractional calculation.

This partially reverts commit 44f406c7893efe092900ca1db7345695837221a4.

Reason for revert: bc isn't available on buildbots.

Bug: 141742445
Test: art/test/testrunner/testrunner.py --target --64 --optimizing -t 670
Change-Id: I961dcaeb9b0cac43e254d9b3f70aaa16f97c1670
2 files changed