Updated VM stacktrace support (20898) with strndup impl.

runtime/platform/c99_support_win.h

Index: runtime/platform/c99_support_win.h
===================================================================
--- runtime/platform/c99_support_win.h	(revision 20895)
+++ runtime/platform/c99_support_win.h	(working copy)
@@ -9,6 +9,7 @@
 // functions. Define them here.
 
 #include <float.h>
+#include <string.h>
 
 static const unsigned __int64 kQuietNaNMask =
     static_cast<unsigned __int64>(0xfff) << 51;
@@ -64,4 +65,18 @@
   }
 }
 
+// size_t used to match function signature on other platforms.
+static inline char* strndup(const char* s, size_t n) {
+  size_t len = strlen(s);
+  if (n < len) {
+    len = n;
+  }
+  char* result = reinterpret_cast<char*>(malloc(len + 1));
+  if (!result) {
+    return NULL;
+  }
+  result[len] = '\0';
+  return reinterpret_cast<char*>(memcpy(result, s, len));
+}
+
 #endif  // PLATFORM_C99_SUPPORT_WIN_H_