[libc] Make two include/llvm-libc-types/ headers more self-contained.

* imaxdiv_t.h needs intmax_t. Get it from <stdint.h> directly
  instead of relying on prior inclusion inside generated inttypes.h.
* include <termios-macros.h> for struct termios to get the #defined
  constant. We do allow inclusion of macro headers from types headers
  in other places.

Author: Alexey Samsonov

libc/include/llvm-libc-types/struct_termios.h

@@ -23,7 +23,6 @@ struct termios {
 #ifdef __linux__
   cc_t c_line; // Line discipline
 #endif         // __linux__
-  // NCCS is defined in llvm-libc-macros/termios-macros.h.
   cc_t c_cc[NCCS]; // Control characters
 #ifdef __linux__
   speed_t c_ispeed; // Input speed