Add sys_hello and testbin/hellotest

`sys_hello` is a new syscall that just prints some text in the terminal.

`testbin/hellotest` is a userland binary that just calls `sys_hello`

userland/testbin/hellotest/Makefile

@@ -0,0 +1,11 @@
+# Makefile for hello
+
+TOP=../../..
+.include "$(TOP)/mk/os161.config.mk"
+
+PROG=hellotest
+SRCS=hello.c
+BINDIR=/testbin
+
+.include "$(TOP)/mk/os161.prog.mk"
+

userland/testbin/hellotest/hello.c

@@ -0,0 +1,8 @@
+#include <unistd.h>
+
+extern int hello(void);
+
+int main() {
+  hello();
+  return 0;
+}