[CRT] Add a generic C version of exp2(f) and use it for all architecture

sdk/lib/crt/math/exp2f.c

@@ -0,0 +1,13 @@
+
+#include <math.h>
+
+_Check_return_
+float
+__cdecl
+exp2f(
+    _In_ float x)
+{
+    /* This below avoids clang to optimize our pow call to exp2 */
+    static const TWO = 2.0f;
+    return powf(TWO, x);
+}