Introduce new convenience variables $_gdb_major and $_gdb_minor

Introduce new convenience variables $_gdb_major and $_gdb_minor

gdb/ChangeLog:
2019-03-30  Eli Zaretskii  <eliz@gnu.org>

	* NEWS: Announce $_gdb_major and $_gdb_minor.

	* top.c (init_gdb_version_vars): New function.
	(gdb_init): Call init_gdb_version_vars.

gdb/testsuite/ChangeLog:
2019-03-30  Simon Marchi <simark@simark.ca>

	* gdb.base/default.exp: Add values for $_gdb_major and
	$_gdb_minor.

gdb/doc/ChangeLog:
2019-03-30  Eli Zaretskii  <eliz@gnu.org>

	* gdb.texinfo (Convenience Vars): Document $_gdb_major and
	$_gdb_minor.