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

Change-Id: Idb39fd04b7a14114801624e744e76e0e7913d951
diff --git a/libc/arch-x86_64/syscalls/__reboot.S b/libc/arch-x86_64/syscalls/__reboot.S
index f4d9245..d62ebcd 100644
--- a/libc/arch-x86_64/syscalls/__reboot.S
+++ b/libc/arch-x86_64/syscalls/__reboot.S
@@ -17,3 +17,4 @@
 1:
     ret
 END(__reboot)
+.hidden _C_LABEL(__reboot)