ZJIT: LIR CCall survivors: Use one stack layout across arches HEADmaster

Previously, the last push, in case the total number of pushes was odd,
ended up in a different location relative to the stack top on x64 versus
A64. On A64, it ends up at `sp[0]`, whereas on x64 it ends up at
`sp[1]`. The difference was due to `CPush` not having consistent
architectural effects in the two arches.

Normalize to `CPushPair` instead. Now A64 will write one more slot, due
to using STP. x64 now uses the two-byte `push 0` instead of the
one-byte `push rdi`. The odd push now always lives at `sp[1]`, leaving
no gap between it and the rest of the survivors. This consistency will
benefit future stack map work.

Also remove some pointless `Vec` allocations.

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