[mlir][sparse] Use the correct ABI on x86 and re-enable tests

c7ec6e19 made LLVM adhere to the x86
psABI and pass bf16 in SSE registers instead of GPRs. This breaks the
custom versions of runtime functions we have for bf16 conversion. A
great fix for this would be to use __bf16 types instead which carry the
right ABI, but that type isn't widely available.

Instead just pretend it's a 32 bit float on the ABI boundary and
carefully cast it to the right type.

Fixes #57042