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

Change-Id: Idb39fd04b7a14114801624e744e76e0e7913d951
diff --git a/libc/arch-x86_64/syscalls/__rt_sigaction.S b/libc/arch-x86_64/syscalls/__rt_sigaction.S
index ee50972..e2892bf 100644
--- a/libc/arch-x86_64/syscalls/__rt_sigaction.S
+++ b/libc/arch-x86_64/syscalls/__rt_sigaction.S
@@ -17,3 +17,4 @@
 1:
     ret
 END(__rt_sigaction)
+.hidden _C_LABEL(__rt_sigaction)