This is split out from the LTO change. It's needed by the LTO build so
that we can easily build these files in non-LTO mode, since they satisfy
calls emitted by LLVM codegen after LTO.
Use wasm's builtin min and max operators to implement libc `fmin`,
`fmax, `fminf`, and `fmaxf`, by handling the NaN cases explicitly.
Credit to https://github.com/emscripten-core/emscripten/pull/9689
for spotting this opportunity!
For functions like sqrt, fabs, and others, use the builtin functions,
which provide single-instruction implementations, rather than using
musl's portable implementations.
