Cache supported hit_bits while setting up the trigger

[en-sc]

Deducing the set of supported hit field values when a trigger is hit is time-consuming and does not provide the user with a diagnostics when the desired timing is not supported.

See riscv/riscv-debug-spec#1080 (comment)

There should also be a configuration parameter to select when to check hit filed support (when setting the trigger or when the trigger is hit) to mitigate the possibility of hit field support changing over time.