dex2oat: Faster .bss and *.rel.ro data collection.

Make this short phase almost 3x faster (36ms -> 13ms),
measured for boot image compilation with `--dump-timings`
on a test device.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I270f7712b3fb1982ab561241a1519aa6c461b90b
5 files changed