arch/x86: Ensure LAPIC mode for exception handler

Attempting to use X2APIC MSRs before the call to enable_lapic()
is made raises exception and double-faults.

Change-Id: Ib97889466af0fbe639bec2be730784acc015b525
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76194
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

src/include/cpu/x86/lapic.h

@@ -184,4 +184,16 @@ void enable_lapic_mode(bool try_set_x2apic);
 void disable_lapic(void);
 void setup_lapic_interrupts(void);
 
+static inline unsigned int early_lapicid(void)
+{
+	if (!CONFIG(SMP))
+		return 0;
+
+	if (!ENV_RAMSTAGE)
+		return 0;
+
+	enable_lapic();
+	return lapicid();
+}
+
 #endif /* CPU_X86_LAPIC_H */