math: test portable FMA even on system with hardware FMA

This makes it a little less likely the portable FMA will be
broken without realizing it.

Change-Id: I7f7f4509b35160a9709f8b8a0e494c09ea6e410a
Reviewed-on: https://go-review.googlesource.com/c/go/+/205337
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/math/all_test.go

@@ -3053,12 +3053,18 @@ func TestYn(t *testing.T) {
 	}
 }
 
+var PortableFMA = FMA // hide call from compiler intrinsic; falls back to portable code
+
 func TestFMA(t *testing.T) {
 	for _, c := range fmaC {
 		got := FMA(c.x, c.y, c.z)
 		if !alike(got, c.want) {
 			t.Errorf("FMA(%g,%g,%g) == %g; want %g", c.x, c.y, c.z, got, c.want)
 		}
+		got = PortableFMA(c.x, c.y, c.z)
+		if !alike(got, c.want) {
+			t.Errorf("PortableFMA(%g,%g,%g) == %g; want %g", c.x, c.y, c.z, got, c.want)
+		}
 	}
 }