dont try to build fork if the arch doesnt support it (like ia64)

libc/sysdeps/linux/common/fork.c

@@ -11,7 +11,9 @@
 #include <unistd.h>
 
 #ifdef __ARCH_HAS_MMU__
+#ifdef __NR_fork
 #define __NR___libc_fork __NR_fork
 _syscall0(pid_t, __libc_fork);
 weak_alias(__libc_fork, fork);
 #endif
+#endif