hostile witness: Is that a concept unique to American Justice?

Not. English jurisprudence.

Usually you are not allowed to cross-examine your own witnesses because they are considered friendly to your cause.

When the court accepts that the witness you are calling is hostile to your cause, usually because the witness is in the payroll of the other side, the rule against cross-examination is lifted.

Calling a hostile witness is a calculated risk.

The witness may possess critical information which can be extracted under oath which will justify the gamble.

BTW "hostile questioning" refers to the questioning of a "hostile", I believe. It does not describe the nature of the questioning itself.

However "hostile" the questioning of Saddam may be, it will be pampering, cooing and cuddling compared to the grotesque barbarities he practised on his own captives.