libc: io: implement file io buffering and internal locking for concurrency

lib/libc/io/stdio/fputc.c

@@ -0,0 +1,27 @@
+#include "file.h"
+
+#include <errno.h>
+#include <stdio.h>
+
+int __fputc(int c, struct __opaque_file *stream)
+{
+	if (stream->f_flags & (FILE_EOF | FILE_ERR)) {
+		return EOF;
+	}
+
+	char cv = c;
+	int ret = __libc_file_write(stream, &cv, 1);
+	if (ret < 1) {
+		return EOF;
+	}
+
+	return c;
+}
+
+int fputc(int c, struct __opaque_file *stream)
+{
+	__libc_file_lock(stream);
+	int ret = __fputc(c, stream);
+	__libc_file_unlock(stream);
+	return ret;
+}