s390x: another batch of intrinsics

[folkertdev]

tracking issue: rust-lang/rust#135681

linking some llvm issues that would let us drop llvm intrinsics when fixed

• s390x: missed optimization to vec_unpackl llvm/llvm-project#129576
• s390x: widening multiplication does not optimize llvm/llvm-project#129705
• s390x: manual vec_subc_u128 is not recognized llvm/llvm-project#129608

This now enables the bigint_helper_methods feature in the standard library. That seems risk-free to me, but it can be worked around if there are good reasons to do so.

[rustbot]

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