internal/abi: set register count constants to zero for regabi experiment

This change sets the register count constants to zero for the
GOEXPERIMENT regabi because currently the users of it (i.e. reflect)
will be broken, since they expect Go functions that implement the new
ABI.

Change-Id: Id3e874c61821a36605eb4e1cccdee36a2759f303
Reviewed-on: https://go-review.googlesource.com/c/go/+/292649
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>

src/internal/abi/abi_amd64.go

@@ -9,12 +9,16 @@ package abi
 const (
 	// See abi_generic.go.
 
+	// Currently these values are zero because whatever uses
+	// them will expect the register ABI, which isn't ready
+	// yet.
+
 	// RAX, RBX, RCX, RDI, RSI, R8, R9, R10, R11.
-	IntArgRegs = 9
+	IntArgRegs = 0 // 9
 
 	// X0 -> X14.
-	FloatArgRegs = 15
+	FloatArgRegs = 0 // 15
 
 	// We use SSE2 registers which support 64-bit float operations.
-	EffectiveFloatRegSize = 8
+	EffectiveFloatRegSize = 0 // 8
 )