Generational code cache roots #282
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This PR ports the generational code cache roots handling code from the LXR branch. See https://github.com/wenyuzhao/mmtk-openjdk/blob/lxr/mmtk/src/gc_work.rs#L242-L262
We split code cache roots into two hash tables, one for nursery roots and one for mature roots. Nursery roots are those added since the previous GC. In a nursery GC, we only scan the nursery code cache roots, assuming mature code cache roots all point to mature objects. In a full-heap GC, we scan both nursery and mature code cache roots.
Because code cache roots are added in bulk in the beginning of execution, and are seldom added later, we expect the number of code cache roots to reduce to zero for most nursery GCs, and greatly reduce the root-scanning time.
This PR also adds USDT tracepoints for code cache roots, and make use of the extensible eBPF timeline tools in mmtk-core introduced in mmtk/mmtk-core#1162.