fix signed char type to be explicitly signed

litex/soc/software/include/base/stdint.h

@@ -16,15 +16,15 @@ typedef unsigned char uint8_t;
 typedef long long int64_t;
 typedef int int32_t;
 typedef short int16_t;
-typedef char int8_t;
+typedef signed char int8_t;
 
-typedef char int_least8_t;
+typedef signed char int_least8_t;
 typedef unsigned char uint_least8_t;
-typedef short int_least16_t;
+typedef signed short int_least16_t;
 typedef unsigned short uint_least16_t;
-typedef int int_least32_t;
+typedef signed int int_least32_t;
 typedef unsigned int uint_least32_t;
-typedef long long int_least64_t;
+typedef signed long long int_least64_t;
 typedef unsigned long long uint_least64_t;
   
 #define __int_c_join(a, b) a ## b