test: guard part of stdlib/StaticBigInt.swift for 32-bit at compile-time

[kateinoigakukun]

To avoid integer literal overflows at compile-time

[benrimmington]

Thanks. This will be fixed by #62746 when the branch is unlocked.
Swift CI no longer uses the watchsimulator-i386 target.
Can it use a wasm32 target to detect this kind of issue, or is that only available externally?

[kateinoigakukun]

Ah, I overlooked it. Thanks.
Unfortunately, wasm32 ci is still only available on our fork. We are continuously trying upstreaming patches to make it possible.