Remove the useless _C_LABEL from generated system calls.

Change-Id: Id1d2fd39972652831ea825f6f9cf940b08f42b5c
diff --git a/libc/arch-x86_64/syscalls/__brk.S b/libc/arch-x86_64/syscalls/__brk.S
index 18c2997..a69f404 100644
--- a/libc/arch-x86_64/syscalls/__brk.S
+++ b/libc/arch-x86_64/syscalls/__brk.S
@@ -14,4 +14,4 @@
 1:
     ret
 END(__brk)
-.hidden _C_LABEL(__brk)
+.hidden __brk