Fix `jl_method_lookup_by_tt` for custom method tables. (#60718)
`jl_mt_assoc_by_type` was hardcoding `jl_method_table` in its fallback
path, causing `jl_method_lookup_by_tt` to incorrectly search the global
method table when given a custom one. This resulted in two issues:
1. Methods in custom MTs weren't found when not already cached (#60716)
2. Global methods were incorrectly cached in custom MT caches, leading
to stale entries after redefinition (#60712)
Fix by passing the method table through to `jl_mt_assoc_by_type` so it
uses the correct MT for fallback lookups.
This behavior is "more strict", since `jl_method_lookup_by_tt` queries
with a method table specified won't return global methods now. Other
areas are not affected by this, e.g., GPUCompiler.jl's inference uses explicit
OverlayMethodTables.
