Implement FMA primops for wasm backend

This is the tracking ticket for implementing fused-multiply-add primops added in !9996 (closed) in the wasm backend's NCG. There is no native FMA on wasm32, but one could easily lower these primops to a ccall to one of fma* functions in libc, subject to the caveats described in !9996 (closed).

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information