Change ARG_MAX/_SC_ARG_MAX back to a constant.

As per the lkml thread https://lkml.org/lkml/2017/11/1/946.

Bug: http://b/65818597
Test: ran tests
Change-Id: I7a0610e6903e6761f2b31416e2f5017bd7a60659
diff --git a/libc/kernel/uapi/linux/limits.h b/libc/kernel/uapi/linux/limits.h
index 41108f9..ad0e33e 100644
--- a/libc/kernel/uapi/linux/limits.h
+++ b/libc/kernel/uapi/linux/limits.h
@@ -20,7 +20,7 @@
 #define _LINUX_LIMITS_H
 #define NR_OPEN 1024
 #define NGROUPS_MAX 65536
-#define _KERNEL_ARG_MAX 131072
+#define ARG_MAX 131072
 #define LINK_MAX 127
 #define MAX_CANON 255
 #define MAX_INPUT 255