compiler: use LLVM intrinsics for math trig operations

I think this failed in the past, but presumably those failures have been
fixed by now. So these intrinsics can now be used.

Using these intrinsics instead of the native Go implementations helps
LLVM to reason about them: it can for example evaluate the value at
compile time or do optimizations like convert
`float32(math.Sin(float64(x)))` into a 32-bit sin operation. If the math
operation is not available on the target platform the C library
implementation will be used instead.

compiler/compiler.go

@@ -878,10 +878,9 @@ func (c *compilerContext) createPackage(irbuilder llvm.Builder, pkg *ssa.Package
 			}
 			// Create the function definition.
 			b := newBuilder(c, irbuilder, member)
-			if _, ok := mathToLLVMMapping[member.RelString(nil)]; ok {
+			if ok := b.defineMathOp(); ok {
 				// The body of this function (if there is one) is ignored and
 				// replaced with a LLVM intrinsic call.
-				b.defineMathOp()
 				continue
 			}
 			if ok := b.defineMathBitsIntrinsic(); ok {