cmd/cgo: fix unaligned arguments typedmemmove crash on iOS

[notatestuser]

On iOS arm64, Irregular typedmemmove and bulkBarrierPreWrite crashes
on unaligned arguments. By aligning the arguments on PtrSize bytes
this is fixed.

The included test attempts to reproduce with
#46893 (comment).

Fixes #46893

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Patch Set 1:

(1 comment)

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Patch Set 3:

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Message from Ian Lance Taylor:

Patch Set 3:

(2 comments)

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Patch Set 4:

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Patch Set 4:

(4 comments)

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