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

Change-Id: Idb39fd04b7a14114801624e744e76e0e7913d951
diff --git a/libc/arch-x86_64/syscalls/__timer_create.S b/libc/arch-x86_64/syscalls/__timer_create.S
index 97f7a56..c4c4a2e 100644
--- a/libc/arch-x86_64/syscalls/__timer_create.S
+++ b/libc/arch-x86_64/syscalls/__timer_create.S
@@ -16,3 +16,4 @@
 1:
     ret
 END(__timer_create)
+.hidden _C_LABEL(__timer_create)