missing user_stack_pointer() instances

for the architectures that have usp in pt_regs and do not have
user_stack_pointer() already defined.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/arch/x86/um/asm/ptrace.h b/arch/x86/um/asm/ptrace.h
index 75513325..54f8102 100644
--- a/arch/x86/um/asm/ptrace.h
+++ b/arch/x86/um/asm/ptrace.h
@@ -86,4 +86,5 @@
 		       unsigned long __user *addr);
 
 #endif
+#define user_stack_pointer(regs) PT_REGS_SP(regs)
 #endif /* __UM_X86_PTRACE_H */