Proof-carrying code: followup improvements

[cfallin]

After initial support is landed for PCC (#6090), we can extend it in a bunch of directions:

• Add PCC flag to fuzzed option set, and resolve any PCC failures (false positives rather than real compiler bugs, hopefully!) as they occur
• Consider turning on PCC by default in Wasmtime for at least for some configurations (e.g., static memories / no bounds-checking, where it is simplest and least likely to have false positives)
• Build a "permissive" mode where errors are logged (to log::warn!, stderr, or some other side-channel?) without failing compilation; perhaps use this for on-by-default deployment before turning on a strict-enforcing mode
• Use PCC facts to verify compilation of table accesses
• Use PCC facts to verify other properties, e.g. invariants of Wasm GC when we have it

[fitzgen]

• Use PCC facts to verify global accesses

(really every load/store other than stack slots should ideally be checked)

[fitzgen]

• support memory64

[fitzgen]

• support non-spectre bounds checks modes