Jit: Optimize block link queries by using hash tables

Repeated erase() + iteration on a std::multimap is extremely slow.

Slow enough that it causes a 7 second long stutter during some
transitions in F-Zero X (a N64 VC game that triggers many, many icache
invalidations).

And slow enough that JitBaseBlockCache::DestroyBlock shows up on a
flame graph as taking >50% of total CPU time on the CPU-GPU thread:
https://i.imgur.com/vvqiFL6.png

This commit optimises those block link queries by replacing the
std::multimap (which is typically implemented with red-black trees)
with hash tables.

Master: https://i.imgur.com/vvqiFL6.png / 7s stutters
(starting from 5.0-2021 and with branch following disabled)

This commit: https://i.imgur.com/hAO74fy.png / ~0.7s stutters, which
is pretty close to 5.0 stable. (5.0-2021 introduced the performance
regression and it is especially noticeable when branch following
is disabled, which is the case for all N64 VC games since 5.0-8377.)

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