crypto/subtle: xor fails when run with race+purego

[randall77]

go test -tags=purego -race -bench=BenchmarkAESCFBEncrypt1K crypto/cipher

This fails with a checkptr fault.

Kinda a false positive, kinda not. checkptr is failing correctly, in that a pointer is generated that straddles two allocations. But that pointer is never used.

Split off from #58785

[gopherbot]

Change https://go.dev/cl/480575 mentions this issue: crypto/subtle: don't cast to *uintptr when word size is 0

[randall77]

@gopherbot please open a backport issue to 1.20. This is a potential crash when running with the race detector.

This code is quite different in 1.19. I don't think it suffers from the same issue.

[gopherbot]

Backport issue(s) opened: #59335 (for 1.19), #59336 (for 1.20).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.