Extend ap_boot_mtx scope to also cover mca_init().

Otherwise, under bootverbose, the lapic_enable_cmc() banner 'lapicX:
CMCI unmasked' is printed by several CPUs in parallel, causing garbled
output for the LAPIC dumps.

Reported by:	royger
Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D15157

sys/x86/x86/mp_x86.c

@@ -1007,10 +1007,10 @@ init_secondary_tail(void)
 	KASSERT(PCPU_GET(idlethread) != NULL, ("no idle thread"));
 	PCPU_SET(curthread, PCPU_GET(idlethread));
 
-	mca_init();
-
 	mtx_lock_spin(&ap_boot_mtx);
 
+	mca_init();
+
 	/* Init local apic for irq's */
 	lapic_setup(1);