[core] log2 polyfill for ARMv5

ARMv5 doesn't have a FPU and apparently the soft code for log2()
is wrong and giving weird results. log() works, so we use a log()
based log2() implementation for ARMv5.

src/mbgl/map/map.cpp

@@ -510,7 +510,7 @@ CameraOptions Map::cameraForLatLngs(const std::vector<LatLng>& latLngs, optional
         scaleY -= (padding->top + padding->bottom) / height;
     }
     double minScale = ::fmin(scaleX, scaleY);
-    double zoom = ::log2(getScale() * minScale);
+    double zoom = util::log2(getScale() * minScale);
     zoom = util::clamp(zoom, getMinZoom(), getMaxZoom());
 
     // Calculate the center point of a virtual bounds that is extended in all directions by padding.

src/mbgl/map/transform_state.cpp

@@ -222,7 +222,7 @@ double TransformState::zoomScale(double zoom) const {
 }
 
 double TransformState::scaleZoom(double s) const {
-    return ::log2(s);
+    return util::log2(s);
 }
 
 double TransformState::worldSize() const {

src/mbgl/util/math.cpp

@@ -21,5 +21,15 @@ uint32_t ceil_log2(uint64_t x) {
     return y;
 }
 
+double log2(double x) {
+// log2() is producing wrong results on ARMv5 binaries
+// running on ARMv7+ CPUs.
+#if defined(__ANDROID__) && defined(__ARM_ARCH_5TE__)
+    return std::log(x) / 0.6931471805599453; // log(x) / log(2)
+#else
+    return ::log2(x);
+#endif
+}
+
 } // namespace util
-} // namespace mbgl
+} // namespace mbgl

src/mbgl/util/math.hpp

@@ -110,5 +110,7 @@ T smoothstep(T edge0, T edge1, T x) {
 // (== number of bits required to store x)
 uint32_t ceil_log2(uint64_t x);
 
+double log2(double x);
+
 } // namespace util
 } // namespace mbgl