cmd/internal/obj/loong64, cmd/link/internal: switch to LoongArch ELF psABI v2 relocs
The LoongArch ELF psABI v2 [1] relocs are vastly simplified from the v1
which involved a stack machine for computing the reloc values, but the
details of PC-relative addressing are changed as well. Specifically, the
`pcaddu12i` instruction is substituted with the `pcalau12i`, which is
like arm64's `adrp` -- meaning the lower bits of a symbol's address now
have to be absolute and not PC-relative.
However, apart from the little bit of added complexity, the obvious
advantage is that only 1 reloc needs to be emitted for every kind of
external reloc we care about. This can mean substantial space savings
(each RELA reloc occupies 24 bytes), and no open-coded stack ops has to
remain any more.
While at it, update the preset value for the output ELF's flags to
indicate the psABI update.
Fixes #58784
[1]: https://loongson.github.io/LoongArch-Documentation/LoongArch-ELF-ABI-EN.html
Change-Id: I5c13bc710eaf58293a32e930dd33feff2ef14c28
Reviewed-on: https://go-review.googlesource.com/c/go/+/455017
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