Remove the useless _C_LABEL from generated system calls.

Change-Id: Id1d2fd39972652831ea825f6f9cf940b08f42b5c
diff --git a/libc/arch-x86_64/syscalls/__reboot.S b/libc/arch-x86_64/syscalls/__reboot.S
index fc8e58d..9b8ef70 100644
--- a/libc/arch-x86_64/syscalls/__reboot.S
+++ b/libc/arch-x86_64/syscalls/__reboot.S
@@ -15,4 +15,4 @@
 1:
     ret
 END(__reboot)
-.hidden _C_LABEL(__reboot)
+.hidden __reboot