x86/enter: Use IBRS on syscall and interrupts

x86/enter: Use IBRS on syscall and interrupts

Set IBRS upon kernel entrance via syscall and interrupts. Clear it
upon exit.

Orabug: 27344012
CVE: CVE-2017-5715

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[Backport: I had to add 'asm/spec_ctrl.h' in the assembler files]
 Also we should not put ENABLE_IBRS on irq_entries_start]

Reviewed-by: John Haxby <john.haxby@oracle.com>
Signed-off-by: Kirtikar Kashyap <kirtikar.kashyap@oracle.com>