aarch64: Add missing LR validation in 'vpaes_cbc_encrypt'

There is an obvious bug there: upon entry to 'vpaes_cbc_encrypt'
LR may get signed. However, on the 'cbc_abort' path the LR is
not going to be unsigned before 'ret' is executed.

Found by manual code inspection.

Co-authored-by: Russ Butler <russ.butler@arm.com>

Change-Id: I646cdfaee28db59aafbbd412d4bb6ba022eff15b
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/49605
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/fipsmodule/aes/asm/vpaes-armv8.pl

@@ -1153,8 +1153,8 @@ vpaes_cbc_encrypt:
 	st1	{v0.16b}, [$ivec]	// write ivec
 
 	ldp	x29,x30,[sp],#16
-	AARCH64_VALIDATE_LINK_REGISTER
 .Lcbc_abort:
+	AARCH64_VALIDATE_LINK_REGISTER
 	ret
 .size	vpaes_cbc_encrypt,.-vpaes_cbc_encrypt