Don't use class-initialization for the owner union

Don't use class-initialization for the owner union

As reported by PhilippeW, valgrind reports that symtab is uninitialized
when compiling with GCC 4.8.5, which is the default compiler on CentOS 7.

This is apparently a compiler bug fixed in later versions, but to keep
CentOS 7 working, this patch initializes the union explicitly instead of
using a class initializer.

gdb/ChangeLog:

2019-11-18  Christian Biesinger  <cbiesinger@google.com>

	* symtab.h (struct symbol) <owner>: Initialize explicitly in the
	constructor instead of using a class initializer.

Change-Id: I94f48afeae5d29cf81a280295e2d02e2d7e1c1f1