add IEEE binary128 long double support to floatscan

just defining the necessary constants:

 LD_B1B_MAX is 2^113 - 1 in base 10^9
 KMAX is 2048 so the x array can hold up to 18432 decimal digits

(the worst case is converting 2^-16495 = 5^16495 * 10^-16495 to
binary, it requires the processing of int(log10(5)*16495)+1 = 11530
decimal digits after discarding the leading zeros, the conversion
requires some headroom in x, but KMAX is more than enough for that)

However this code is not optimal on archs with IEEE binary128
long double because the arithmetics is software emulated (on
all such platforms as far as i know) which means big and slow
strtod.

src/internal/floatscan.c

@@ -15,12 +15,20 @@
 #define LD_B1B_MAX 9007199, 254740991
 #define KMAX 128
 
-#else /* LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384 */
+#elif LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384
 
 #define LD_B1B_DIG 3
 #define LD_B1B_MAX 18, 446744073, 709551615
 #define KMAX 2048
 
+#elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384
+
+#define LD_B1B_DIG 4
+#define LD_B1B_MAX 10384593, 717069655, 257060992, 658440191
+#define KMAX 2048
+
+#else
+#error Unsupported long double representation
 #endif
 
 #define MASK (KMAX-1)