arm64: Fix purecap efi_rt_arch_call_nofault not preserving DAIF

x16 is IP0, a call-clobbered register; call-preserved registers start at
x19, so use that and actually restore DAIF properly.

Fixes:	260b5a0 ("efirt: Shim AArch64 interface for pure-capability Morello kernels")

sys/arm64/arm64/efirt_support.S

@@ -97,7 +97,7 @@ ENTRY(efi_rt_arch_call_nofault)
 	 * Temporarily disable all interrupts as we can't handle them whilst
 	 * CSP is clobbered during the EFI call.
 	 */
-	mrs	x16, daif
+	mrs	x19, daif
 	msr	daifset, #(DAIF_ALL)
 
 	/*
@@ -120,7 +120,7 @@ ENTRY(efi_rt_arch_call_nofault)
 	mov	csp, c8
 
 	/* Restore interrupt mask */
-	msr	daif, x16
+	msr	daif, x19
 
 	/* Done with the almighty DDC */
 	msr	ddc, czr