gcm_ghash_v8: Simplification

In the context of the change, t0 and IN are the same after

```
veor		$IN,$t0,$t2		@ inp^=Xi
veor		$t1,$t0,$t2		@ $t1 is rotated inp^Xi
```

Moreover, after all of

```
vpmull2.p64	$Xl,$H,$IN		@ H.lo·Xi.lo
vext.8          $IN, $IN, $IN, aws#8

veor		$t1,$t1,$IN		@ Karatsuba pre-processing
vpmull.p64	$Xm,$Hhl,$t1		@ (H.lo+H.hi)·(Xi.lo+Xi.hi)

vext.8          $IN, $IN, $IN, aws#8
```

`IN` is unchanged because it was swapped twice, and t1 only feeds
into the computation of Xm and is not used further afterwards.

Hence, the above is equivalent to

```
vpmull2.p64	$Xl,$H,$IN		@ H.lo·Xi.lo
vext.8          $t1, $IN, $IN, aws#8

veor		$t1,$t1,$IN		@ Karatsuba pre-processing
vpmull.p64	$Xm,$Hhl,$t1		@ (H.lo+H.hi)·(Xi.lo+Xi.hi)
```

removing one `vext`.

crypto/fipsmodule/modes/asm/ghashv8-armx.pl

@@ -484,15 +484,13 @@
 .Lodd_tail_v8:
 	vext.8		$t2,$Xl,$Xl,#8
 	veor		$IN,$t0,$t2		@ inp^=Xi
-	veor		$t1,$t0,$t2		@ $t1 is rotated inp^Xi
 
         vpmull2.p64	$Xl,$H,$IN		@ H.lo·Xi.lo
-        vext.8          $IN, $IN, $IN, #8
+        vext.8          $t1, $IN, $IN, #8
 
 	veor		$t1,$t1,$IN		@ Karatsuba pre-processing
 	vpmull.p64	$Xm,$Hhl,$t1		@ (H.lo+H.hi)·(Xi.lo+Xi.hi)
 
-        vext.8          $IN, $IN, $IN, #8
         vpmull.p64	$Xh,$H,$IN		@ H.hi·Xi.hi
         vext.8          $IN, $IN, $IN, #8