cmd/internal/obj/x86: use ABI scratch registers for WRAPPER prologue

Currently the prologue generated for WRAPPER assembly functions uses BX
and DI, but these are argument registers in the register-based calling
convention. Thus, these end up being clobbered when we want to have an
ABIInternal assembly function.

Define REGENTRYTMP0 and REGENTRYTMP1, aliases for the dedicated function
entry scratch registers R12 and R13, and use those instead.

For #40724.

Change-Id: Ica78c4ccc67a757359900a66b56ef28c83d88b3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/303314


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>