x86, mce: rename static variables around trigger

"trigger" is not straight forward name for valiable that holds name
of user mode helper program which triggered by machine check events.

This patch renames this valiable and kins to more recognizable names.

	trigger		=> mce_helper
	trigger_argv	=> mce_helper_argv
	notify_user	=> mce_need_notify

No functional changes.

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>