ART: Make kDebugVerifier a verbose flag

To aid in debugging efforts, finally introduce a verifier-debug
verbose-logging flag that takes over the compile-time kDebugVerifier
flag.

Collecting verification times from a well-known large app during
dex2oat via dump-timings shows a difference in the noise.

Logging of the steps is restricted to failure cases, to avoid
spam.

Test: m
Test: m test-art-host
Test: manual: dex2oat --runtime-arg -verbose:verifier-debug APK with failures
Change-Id: Icb998cc42a531b80f20f986625869505271b7c98