libm: Provide log2f

Author: jepler

lib/libm/math.c

@@ -50,6 +50,9 @@ float copysignf(float x, float y) {
 
 static const float _M_LN10 = 2.30258509299404; // 0x40135d8e
 float log10f(float x) { return logf(x) / (float)_M_LN10; }
+#undef _M_LN2
+static const float _M_LN2 = 0.6931472;
+float log2f(float x) { return logf(x) / (float)_M_LN2; }
 
 float tanhf(float x) {
     if (isinf(x)) {