add kernel_string(addr) built-in function

kernel_string built-in function is purpose for
access kernel string.

One example to use this kernel_string(tested in x86-64):

trace probe:do_execve filename=%di {
	printf("[do_execve entry]: (%s) name=%s\n",
		execname(), kernel_string(arg2))
}

Result:

[do_execve entry]: (bash) name=/usr/bin/cat
[do_execve entry]: (bash) name=/usr/bin/ls
[do_execve entry]: (bash) name=/usr/bin/top
...

Signed-off-by: Tadaki SAKAI <stadaki.dev@gmail.com>
Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>
 (Jovi: use kp_tstring_new_local instead of kp_tstring_new)