powerpc64: Clear FSCR SPR, so that it's in a known state

This now turns any access to the DSCR SPR into a SIGILL.  Later commits will
make DCSR work correctly on POWER8 and POWER9.

PR:		237208

sys/powerpc/aim/mp_cpudep.c

@@ -98,6 +98,12 @@ cpudep_ap_early_bootstrap(void)
 
 			mtspr(SPR_LPCR, lpcr);
 			isync();
+
+			/*
+			 * Nuke FSCR, to be managed on a per-process basis
+			 * later.
+			 */
+			mtspr(SPR_FSCR, 0);
 		}
 #endif
 		break;

sys/powerpc/powerpc/cpu.c

@@ -663,6 +663,9 @@ cpu_powerx_setup(int cpuid, uint16_t vers)
 	if ((mfmsr() & PSL_HV) == 0)
 		return;
 
+	/* Nuke the FSCR, to disable all facilities. */
+	mtspr(SPR_FSCR, 0);
+
 	/* Configure power-saving */
 	switch (vers) {
 	case IBMPOWER8: