scripts/kallsyms: fix wrong kallsyms_relative_base

There is the code in the read_symbol function in 'scripts/kallsyms.c':

	if (is_ignored_symbol(name, type))
		return NULL;

	/* Ignore most absolute/undefined (?) symbols. */
	if (strcmp(name, "_text") == 0)
		_text = addr;

But the is_ignored_symbol function returns true for name="_text" and
type='A'. So the next condition is not executed and the _text variable
is always zero.

It makes the wrong kallsyms_relative_base symbol as a result of the code
(CONFIG_KALLSYMS_BASE_RELATIVE is defined):

	if (base_relative) {
		output_label("kallsyms_relative_base");
		output_address(relative_base);
		printf("\n");
	}

Because the output_address function uses the _text variable.

So the kallsyms_lookup function and all related functions in the kernel
do not work properly. For example, the stack trace in oops:

 Call Trace:
 [aa095e58] [809feab8] kobj_ns_ops_tbl+0x7ff09ac8/0x7ff1c1c4 (unreliable)
 [aa095e98] [80002b64] kobj_ns_ops_tbl+0x7f50db74/0x80000010
 [aa095ef...