Fix undefined reference to mingw (#10)

Author: eukarpov

libgfortran/libgfortran.h

@@ -67,8 +67,14 @@ extern float __strtof (const char *, char **);
 #define gfc_strtof __strtof
 extern double __strtod (const char *, char **);
 #define gfc_strtod __strtod
+
+#if defined(__aarch64__)
+#define gfc_strtold strtold
+#else
 extern long double __strtold (const char *, char **);
 #define gfc_strtold __strtold
+#endif
+
 #else
 #define gfc_strtof strtof
 #define gfc_strtod strtod