Use hidden visibility for generated stubs whose names begin with underscores.

Change-Id: Idb39fd04b7a14114801624e744e76e0e7913d951
diff --git a/libc/arch-x86_64/syscalls/__arch_prctl.S b/libc/arch-x86_64/syscalls/__arch_prctl.S
index 70f284a..0adcfd2 100644
--- a/libc/arch-x86_64/syscalls/__arch_prctl.S
+++ b/libc/arch-x86_64/syscalls/__arch_prctl.S
@@ -16,3 +16,4 @@
 1:
     ret
 END(__arch_prctl)
+.hidden _C_LABEL(__arch_prctl)