x86, mce: Get rid of TIF_MCE_NOTIFY and associated mce tricks

We now switch to the kernel stack when a machine check interrupts
during user mode.  This means that we can perform recovery actions
in the tail of do_machine_check()

Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>