ART: make generational CC a runtime option

Add -Xgc:[no]generational_cc option to control whether generational
Concurrent Copy collector is enabled when art is launched.

Repurpose the environment variable and preprocessor macro name
`ART_USE_GENERATIONAL_CC` to selecting the default behavior (if unset,
the default is to use Generational CC collection by default).

Test: Run art with -Xgc:[no]generational_cc along with -XX:DumpGCPerformanceOnShutdown on some benchmarks to see if minor GC runs. parsed_options_test is updated to test this new option.
Test: make test-art-host-gtest-parsed_options_test
Test: make test-art-target-gtest-parsed_options_test
Bug: 112187497
Bug: 67628039
Change-Id: I6b4660b06f7e793911a2601bed24c33777190741