Use wasm math opcodes in wasm NCG

Currently, the wasm NCG lowers Cmm math MachOps like MO_F64_Sqrt to call their respective functions in libc. Some of these operations are actually available as a single wasm opcode (e.g. f64.sqrt). The wasm NCG should lower to a single math opcode whenever possible.

https://github.com/WebAssembly/wasi-libc/blob/main/libc-bottom-half/sources/math/math-builtins.c is a good reference for what math functions are available as a single wasm opcode. wasi-libc already takes advantage of these opcodes, but it's still nice to avoid the function call overhead when possible.