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

Change-Id: Idb39fd04b7a14114801624e744e76e0e7913d951
diff --git a/libc/arch-x86_64/syscalls/__openat.S b/libc/arch-x86_64/syscalls/__openat.S
index 022bf7f..36f7ddc 100644
--- a/libc/arch-x86_64/syscalls/__openat.S
+++ b/libc/arch-x86_64/syscalls/__openat.S
@@ -17,3 +17,4 @@
 1:
     ret
 END(__openat)
+.hidden _C_LABEL(__openat)