cmd/internal/obj/x86: simplify huge frame prologue

For stack frames larger than StackBig, the stack split prologue needs
to guard against potential wraparound. Currently, it carefully
arranges to avoid underflow, but this is complicated and requires a
special check for StackPreempt. StackPreempt is no longer the only
stack poison value, so this check will incorrectly succeed if the
stack bound is poisoned with any other value.

This CL simplifies the logic of the check, reduces its length, and
accounts for any possible poison value by directly checking for
underflow.

Change-Id: I917a313102d6a21895ef7c4b0f304fb84b292c81
Reviewed-on: https://go-review.googlesource.com/c/go/+/307010


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