Add long double printf formatting support

You can now easily print numbers that have 80 bits of precision. This
change also imports many of the test vectors from the gdtoa codebase.

third_party/gdtoa/gdtoa.h

@@ -3,6 +3,26 @@
 #if !(__ASSEMBLER__ + __LINKER__ + 0)
 COSMOPOLITAN_C_START_
 
+/**
+ * Configures g_*fmt()
+ *
+ * @param ic
+ * 	0 ==> Infinity or NaN
+ * 	1 ==> infinity or nan
+ * 	2 ==> INFINITY or NAN
+ * 	3 ==> Inf or NaN
+ * 	4 ==> inf or nan
+ * 	5 ==> INF or NAN
+ * @param ic determines if NaNs are rendered as NaN(...)
+ * 	0 ==> no
+ * 	1 ==> yes
+ * 	2 ==> no for default NaN values; yes otherwise
+ * @param ns determines sign of NaN values reported
+ * 	0 ==> distinguish NaN and -NaN
+ * 	1 ==> report both as NaN
+ */
+#define NIK(ic, nb, ns) (ic + 6 * (nb + 3 * ns))
+
 enum {
   /* return values from strtodg */
   STRTOG_Zero = 0,