Auto merge of #118344 - saethlin:rmeta-header-pos, r=WaffleLapkin

 Use a u64 for the rmeta root position

Waffle noticed this in rust-lang/rust#117301 (comment)

We've upgraded the other file offsets to u64, and this one only costs 4 bytes per file. Also the way the truncation was being done before was extremely easy to miss, I sure missed it! It's not clear to me if not having this change effectively made the other upgrades from u32 to u64 ineffective, but we can have it now.

r? `@WaffleLapkin`

Author: bors