Revert "update static void parse_number()"

because:
- Constructing fp numbers isn't quite trivial
- 3 additional calls to strchr per number is noticeable slow

Also: exclude abitest.c:ret_longdouble_test2 on _WIN32
for mixed gcc/tcc scenario

test case:
- make -k test (on win32):
  -2.120000 0.500000 23000000000.000000
  +2.120000 0.500000 22999999999.999996
  ...
  ret_longdouble_test2... failure

This reverts 857f7dbfa6
and deaee6c249

tests/abitest.c

@@ -486,7 +486,10 @@ int main(int argc, char **argv) {
   RUN_TEST(ret_2float_test);
   RUN_TEST(ret_2double_test);
   RUN_TEST(ret_longlong_test2);
+#if !defined _WIN32 || !defined __GNUC__
+  /* on win32, 'long double' is 10-byte with gcc, but is 'double' with tcc/msvc */
   RUN_TEST(ret_longdouble_test2);
+#endif
   RUN_TEST(reg_pack_test);
   RUN_TEST(reg_pack_longlong_test);
   RUN_TEST(sret_test);