Add option to parse version script during linking.

1) Add option to parse version script file specifying exported symbols and
variables during link time.
2) Added test version script file.
3) Minor style cleanup.
4) Remove HandleVTable as it is no longer needed.
5) header-abi-diff has a mode to enable advice-only mode. This means no
failures on abi extensions / incompatiblity. Only report generation.
5) Seperate out functionality into util lib.

Test: 1) header-abi-dumper -o example1.dump tests/input/example1.cpp -I
tests -- -x c++ -std=c++11 -I . -I
~/android/aosp/external/clang/lib/Headers

2) header-abi-linker -o test.lsdump example1.dump -v
tests/input/test_version_script.map -arch arm64 -api 24,
test.lsdump has only the symbols specified by test_version_script.amp

Change-Id: Ib539db4a886abe4917e09cd8643ab35ffecb0df1