[FP16] Implement madd and nmadd.

[brendandahl]

Specified at https://github.com/WebAssembly/half-precision/blob/main/proposals/half-precision/Overview.md

A few notes:

• The F32x4 and F64x2 versions of madd and nmadd are missing spect tests.
• For madd, the implementation was incorrectly doing (b*c)+a where it should be (a*b)+c.
• For nmadd, the implementation was incorrectly doing (-b*c)+a where it should be -(a*b)+c.
• There doesn't appear to be a great way to actually implement a fused nmadd, but the spec allows the double rounded version I added.