Fix the type of u_ar0 in <sys/user.h>.

(cherry picked from commit e03950fa0c5567edf70d011b856a027e03b1c0f7)

Bug: 18172268
Change-Id: I0feda6b253882f68f47bcf30fad998286cc7f620
diff --git a/libc/include/sys/user.h b/libc/include/sys/user.h
index 66b371d..0e36825 100644
--- a/libc/include/sys/user.h
+++ b/libc/include/sys/user.h
@@ -91,7 +91,7 @@
   unsigned long start_stack;
   long int signal;
   int reserved;
-  unsigned long u_ar0;
+  struct user_regs_struct* u_ar0;
   struct user_fpregs_struct* u_fpstate;
   unsigned long magic;
   char u_comm[32];
@@ -155,7 +155,7 @@
   long int signal;
   int reserved;
   int pad1;
-  unsigned long u_ar0;
+  struct user_regs_struct* u_ar0;
   struct user_fpregs_struct* u_fpstate;
   unsigned long magic;
   char u_comm[32];
@@ -175,7 +175,7 @@
   unsigned long start_data;
   unsigned long start_stack;
   long int signal;
-  unsigned long u_ar0;
+  void* u_ar0;
   unsigned long magic;
   char u_comm[32];
 };