intrinsics.fmuladdf{16,32,64,128}: expose llvm.fmuladd.* semantics

Add intrinsics `fmuladd{f16,f32,f64,f128}`. This computes `(a * b) +
c`, to be fused if the code generator determines that (i) the target
instruction set has support for a fused operation, and (ii) that the
fused operation is more efficient than the equivalent, separate pair
of `mul` and `add` instructions.

https://llvm.org/docs/LangRef.html#llvm-fmuladd-intrinsic

MIRI support is included for f32 and f64.

The codegen_cranelift uses the `fma` function from libc, which is a
correct implementation, but without the desired performance semantic. I
think this requires an update to cranelift to expose a suitable
instruction in its IR.

I have not tested with codegen_gcc, but it should behave the same
way (using `fma` from libc).

compiler/rustc_span/src/symbol.rs

@@ -914,6 +914,10 @@ symbols! {
         fmt_debug,
         fmul_algebraic,
         fmul_fast,
+        fmuladdf128,
+        fmuladdf16,
+        fmuladdf32,
+        fmuladdf64,
         fn_align,
         fn_delegation,
         fn_must_use,