Remove the useless _C_LABEL from generated system calls.

Change-Id: Id1d2fd39972652831ea825f6f9cf940b08f42b5c
diff --git a/libc/arch-mips64/syscalls/__exit.S b/libc/arch-mips64/syscalls/__exit.S
index 5adcc5b..dac53b9 100644
--- a/libc/arch-mips64/syscalls/__exit.S
+++ b/libc/arch-mips64/syscalls/__exit.S
@@ -23,4 +23,4 @@
     move ra, t0
     .set pop
 END(__exit)
-.hidden _C_LABEL(__exit)
+.hidden __exit