abi: fix aggregate passed by regs 2nd reg offset

It was broken for example `struct { i32 a; f64 b; }` (would try to
load/store b from byte offset 4, not 8). Introduce r2off, realize in
x86-64 it's always 8; even `struct {i32 a; f32 b;}` gets passed in one
(integer) register. But not so in (future) ABIs like RISC-V, I believe
there `{i32, f32}` would get passed in 1 integer and 1 float register
(r2off = 4).

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