POSIX: Use NV instead of hardcoded 'double' in strtol()/strtoul().

Casting (unsigned) long value to 'double' might cause unnecessary
loss of precision if double's significand is not enough wide to preserve
(unsigned) long and NV is configured to be wider than double.

ext/POSIX/POSIX.xs

@@ -3549,7 +3549,7 @@ strtol(str, base = 0)
             num = strtol(str, &unparsed, base);
 #if IVSIZE < LONGSIZE
             if (num < IV_MIN || num > IV_MAX)
-                PUSHs(sv_2mortal(newSVnv((double)num)));
+                PUSHs(sv_2mortal(newSVnv((NV)num)));
             else
 #endif
                 PUSHs(sv_2mortal(newSViv((IV)num)));
@@ -3583,7 +3583,7 @@ strtoul(str, base = 0)
             num = strtoul(str, &unparsed, base);
 #if UVSIZE < LONGSIZE
             if (num > UV_MAX)
-                PUSHs(sv_2mortal(newSVnv((double)num)));
+                PUSHs(sv_2mortal(newSVnv((NV)num)));
             else
 #endif
                 PUSHs(sv_2mortal(newSVuv((UV)num)));