Separate art::Exec from utils

The rest of utils.cc does not depend on art::Runtime. This separates
the part dependent on that class, so that including utils.cc in the
build does not require the entire Runtime. Another preparatory cleanup
to getting tools to build on Windows.

Bug: 22322814
Test: test-art
Change-Id: I194ff363fc2ab87e5311ecea6973a2d0fad2621d