Add basic runtime-plugins support.

This allows one to pass shared-libraries on the command line that the
runtime will load as plugins. They have access to runtime code and can
install hooks to add functionality. Currently the only hook they can
touch is JavaVMExt::AddEnvironmentHook to register a callback for
GetEnv(). More hooks might be added in the future.

Test: ./test/run-test 900
Change-Id: I852b4daf5a3fa71e9888722bc07794632c0e5010