[PowerPC] Ignore fortify-source warning in trap vector copy

Due to a bug in clang 9.0.0 source tracking, the trap vector copying will
always trigger a fortify-source warning.

The destination buffers are 0x2f00 bytes, and the bcopy region is 0x2e00
bytes, so there is not an overflow here.

(I have been running with this patch since September.)

sys/powerpc/ofw/ofw_machdep.c

@@ -87,6 +87,9 @@ char		save_trap_of[0x2f00];            /* EXC_LAST */
 int		ofwcall(void *);
 static int	openfirmware(void *args);
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wfortify-source"
+
 __inline void
 ofw_save_trap_vec(char *save_trap_vec)
 {
@@ -107,6 +110,8 @@ ofw_restore_trap_vec(char *restore_trap_vec)
 	__syncicache((void *)PHYS_TO_DMAP(EXC_RSVD), EXC_LAST - EXC_RSVD);
 }
 
+#pragma clang diagnostic pop
+
 /*
  * Saved SPRG0-3 from OpenFirmware. Will be restored prior to the callback.
  */