arm64: Stop trashing the padding after boot_el

When building struct arm64_bootparams to pass to initarm we store the
boot_el field as a 64-bit value, however it is defined as an int which
is 32-bits.

Switch to store using the 'w' register as this will store a correctly
sized value.

Previously this would trash the trailing padding, so is only a
correctness issue.

Reviewed by:	emaste
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D58988

sys/arm64/arm64/locore.S

@@ -156,7 +156,7 @@ virtdone:
 	add	x25, x25, :lo12:initstack
 	str	x25, [x0, #BP_KERN_STACK]
 	str	x27, [x0, #BP_KERN_TTBR0]
-	str	x23, [x0, #BP_BOOT_EL]
+	str	w23, [x0, #BP_BOOT_EL]
 
 	/* Set these before they are used in kasan_init_early */
 	adrp	x1, pmap_sh_attr