fstat: make new code aarch64 specific

The new code get's used by MIPS64 N64 and fails.
Make the new code aarch64 specific.

libc/sysdeps/linux/common/fstat.c

@@ -21,7 +21,7 @@ int fstat(int fd, struct stat *buf)
 }
 libc_hidden_def(fstat)
 
-#elif __WORDSIZE == 64 && defined __NR_newfstatat
+#elif __WORDSIZE == 64 && defined __NR_newfstatat && __aarch64__
 #include <fcntl.h>
 
 int fstat(int fd, struct stat *buf)