fix error in constraints for static link libc init barrier

commit 4390383b32 inadvertently used "r"
instead of "0" for the input constraint, which only happened to work
for the configuration I tested it on because it usually makes sense
for the compiler to choose the same input and output register.

src/env/__libc_start_main.c

@@ -80,7 +80,7 @@ int __libc_start_main(int (*main)(int,char **,char **), int argc, char **argv)
 	/* Barrier against hoisting application code or anything using ssp
 	 * or thread pointer prior to its initialization above. */
 	int (*stage2)();
-	__asm__ ( "" : "=r"(stage2) : "r"(libc_start_main_stage2) : "memory" );
+	__asm__ ( "" : "=r"(stage2) : "0"(libc_start_main_stage2) : "memory" );
 	return stage2(main, argc, argv);
 }