math: fix Log2 test failures on ppc64 (and s390)

- Make Log2 exact for powers of two.
- Fix error tolerance function to make tolerance
  a function of the correct (expected) value.

Fixes #9066.

Change-Id: I0320a93ce4130deed1c7b7685627d51acb7bc56d
Reviewed-on: https://go-review.googlesource.com/12230Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/math/all_test.go

@@ -1730,8 +1730,10 @@ func tolerance(a, b, e float64) bool {
 		d = -d
 	}
 
-	if a != 0 {
-		e = e * a
+	// note: b is correct (expected) value, a is actual value.
+	// make error tolerance a fraction of b, not a.
+	if b != 0 {
+		e = e * b
 		if e < 0 {
 			e = -e
 		}

src/math/log10.go

@@ -18,5 +18,10 @@ func Log2(x float64) float64
 
 func log2(x float64) float64 {
 	frac, exp := Frexp(x)
+	// Make sure exact powers of two give an exact answer.
+	// Don't depend on Log(0.5)*(1/Ln2)+exp being exactly exp-1.
+	if frac == 0.5 {
+		return float64(exp - 1)
+	}
 	return Log(frac)*(1/Ln2) + float64(exp)
 }