fix a few write barrier issues (#62687)
`jl_genericmemory_copyto` had the write barrier after instead of before
which is invalid for concurrent GC. Also, a number of manual `jl_gc_wb`
were added which should have just been `jl_gc_write[_atomic]`
found by claude.
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Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
