MJIT: Change default --mjit-max-cache back to 100

These days we benchmark MJIT using yjit-bench. The warmup duration in
yjit-bench is very short, so compiling many methods comes at a cost even
while it's actually optimal for MJIT to compile everything / tens of
thousands of methods once it reaches the peak performance.

yjit-bench doesn't necessarily represent the peak performance on production.
It measures the performance of Ruby 30~60s after boot. If your JIT takes
more than 1 minute to warm up, there's no way for the JIT to make the numbers
good on yjit-bench.

Until we make MJIT's compilation much faster, we don't afford compiling
10,000 methods on yjit-bench.

This change alone makes MJIT's benchmark number on railsbench 2x better :p

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