Fix undefined behavior

Transmuting from an immutable to a mutable reference is undefined
behavior in Rust, with the exception of UnsafeCell which tells the
optimizer to not draw too many conclusions. Sadly UnsafeCell::get cannot
yet be used in statics (see https://github.com/rust-lang/rust/issues/51718),
so this works around that by making the statics into functions and
making C macros to call those.

include/bits/stdio.h

@@ -2,6 +2,9 @@
 #define _BITS_STDIO_H
 
 #define EOF (-1)
+#define stdin __stdin()
+#define stdout __stdout()
+#define stderr __stderr()
 
 int fprintf(FILE * stream, const char * fmt, ...);
 int printf(const char * fmt, ...);