math/big: temporarily disable buggy shlVU assembly for arm64

This addresses the failures we have seen in #31084. The correct
fix is to find the actual bug in the assembly code.

Updates #31084.

Change-Id: I437780c53d0c4423d742e2e3b650b899ce845372
Reviewed-on: https://go-review.googlesource.com/c/go/+/169721
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/big/arith_arm64.s

@@ -197,6 +197,10 @@ len0:
 
 // func shlVU(z, x []Word, s uint) (c Word)
 TEXT ·shlVU(SB),NOSPLIT,$0
+	// Disable assembly for now - it is subtly incorrect.
+	// See #31084 for a test that fails using this code.
+	B	·shlVU_g(SB)
+
 	MOVD	z+0(FP), R0
 	MOVD	z_len+8(FP), R1
 	MOVD	x+24(FP), R2