Define char8_t conditionally

Following up on commit d55527c, this
patch also puts the `char8_t` typedef under a macro check.

From C++20, `char8_t` is a builtin type, so in that case the typedef
must not be present. The C++ spec has a macro specific to check if
`char8_t` is builtin. This macro is called `__cpp_char8_t`.

Author: vhscampos

newlib/libc/include/uchar.h

@@ -51,7 +51,9 @@ typedef _mbstate_t mbstate_t;
 #define _MBSTATE_DECLARED
 #endif
 
+#ifndef __cpp_char8_t
 typedef unsigned char char8_t;
+#endif
 #if !defined __cplusplus || __cplusplus < 201103L
 typedef __uint_least16_t char16_t;
 typedef __uint_least32_t char32_t;