[MLIR][Arith] Add rounding mode attribute to `truncf` (#86152)

Add rounding mode attribute to `arith`. This attribute can be used in
different FP `arith` operations to control rounding mode. Rounding modes
correspond to IEEE 754-specified rounding modes. Use in `arith.truncf` folding.

As this is not supported in dialects other than LLVM, conversion should fail for
now in case this attribute is present.

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Signed-off-by: Victor Perez <victor.perez@codeplay.com>