revert hacks for types of stdint.h integer constant macros

per 7.18.4: Each invocation of one of these macros shall expand to an
integer constant expression suitable for use in #if preprocessing
directives. The type of the expression shall have the same type as
would an expression of the corresponding type converted according to
the integer promotions. The value of the expression shall be that of
the argument.

the key phrase is "converted according to the integer promotions".
thus there is no intent or allowance that the expression have
smaller-than-int types.

include/stdint.h

@@ -85,13 +85,13 @@ typedef unsigned long long uintmax_t;
 
 #if !defined __cplusplus || defined __STDC_CONSTANT_MACROS
 
-#define INT8_C(c)  ((int8_t) + (c))
-#define INT16_C(c) ((int16_t) + (c))
+#define INT8_C(c)  c
+#define INT16_C(c) c
 #define INT32_C(c) c
 #define INT64_C(c) c ## LL
 
-#define UINT8_C(c)  ((uint8_t) + (c))
-#define UINT16_C(c) ((uint16_t) + (c))
+#define UINT8_C(c)  c ## U
+#define UINT16_C(c) c ## U
 #define UINT32_C(c) c ## U
 #define UINT64_C(c) c ## ULL