[GR-78810] Various GC-related fixes.

[graalvmbot]

• Recognize G1 card-table addresses in heap location diagnostics.
• Move the G1 DisableExplicitGC handling into the SVM runtime so that it doesn't interfer with other forced collections (e.g., heap dumping or dcmd).
• Fix potential crashes with serial crash (object constants that are embedded in code need to be visited while the code is still reachable through its tether).
• Fix GCRunDCmd so that it triggers a full GC.
• Store the requested collection strength in GCCause and add explicit WhiteBox GC causes.
• Replace GC.collect(...) and GC.collectCompletely(...) with a single cause-driven collect interface.