fix rounding of hex floats in strtod/scanf on ld64 and ld128 archs

the expression LDBL_MANT_DIG/4+1 was intended to represent the number
of hex digits which can be significant, after which all that matters
to the result, regardless of rounding mode, is whether any part of the
tail is nonzero.

however, the expression LDBL_MANT_DIG/4 is only exact for ld80 archs.
for ld64 and ld128, the truncation of the remainder caused one too few
digits to be processed, producing incorrect rounding.

for ld128 archs, only strtold and scanf's %La conversion specifier
were affected; doubles and floats still had plenty of trailing digits
to yield a correct final rounding. but for ld64 archs, the number of
digits processed were insufficient for double, and could affect any
code using strtod or scanf's %a.

for ld64 archs, 0x1.111111111111281 produces an incorrect rounding
down, and 0x1.11111111111111 produces an incorrect rounding up.

for ld128 archs, the inputs 0x1.111111111111111111111111111281 and
0x1.11111111111111111111111111111 behave respectively.

rounding up in the expression for the number of hex digits needed
produces correct results.

src/internal/floatscan.c

@@ -347,7 +347,7 @@ static long double hexfloat(FILE *f, int bits, int emin, int sign, int pok)
 			else d = c-'0';
 			if (dc<8) {
 				x = x*16 + d;
-			} else if (dc < LDBL_MANT_DIG/4+1) {
+			} else if (dc < (LDBL_MANT_DIG+3)/4+1) {
 				y += d*(scale/=16);
 			} else if (d && !gottail) {
 				y += 0.5*scale;