This patch extends the Math-to-EmitC conversion to cover `math.roundeven` and
`math.sqrt` for scalar f32/f64 values.
`math.roundeven` and `math.sqrt` are lowered to `emitc.call_opaque` using the
appropriate target-specific names:
C: `roundevenf` / `roundeven`, `sqrtf` / `sqrt`
C++: `std::roundeven`, `std::sqrt`
The patch also adds coverage for `math.rsqrt`. There is no direct EmitC
lowering for `math.rsqrt`; instead, the new tests verify the existing expansion
path through `-math-expand-ops=ops=rsqrt`, followed by `-convert-math-to-emitc`
and `-convert-arith-to-emitc`. This ensures the cross-dialect lowering sequence
produces the expected `emitc.constant`, `emitc.call_opaque`, and `emitc.div`
operations for f32/f64.
Unsupported cases remain unchanged. In particular, the new test documents that
f16 math.rsqrt is not lowered because math.sqrt is only converted for f32/f64.
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