tools/nolibc: implement strerror()

strerror() is commonly used.
For example in kselftest which currently needs to do an #ifdef NOLIBC to
handle the lack of strerror().

Keep it simple and reuse the output format of perror() for strerror().

Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

tools/include/nolibc/stdio.h

@@ -376,6 +376,16 @@ int setvbuf(FILE *stream __attribute__((unused)),
 	return 0;
 }
 
+static __attribute__((unused))
+const char *strerror(int errno)
+{
+	static char buf[18] = "errno=";
+
+	i64toa_r(errno, &buf[6]);
+
+	return buf;
+}
+
 /* make sure to include all global symbols */
 #include "nolibc.h"