stack overflow (morestack).

For example, deferproc might call malloc, which does one of the
above checks (without allocating a full frame), which might trigger
a call to morestack.  This sequence needs to fit in the bottom
section of the stack.  On amd64, morestack's frame is 40 bytes, and
deferproc's frame is 56 bytes.  That fits well within the
StackGuard - StackSmall bytes at the bottom.

	bls.To.SetTarget(movw)

	// BL runtime.morestack
	call := obj.Appendp(movw, c.newprog)
	call.As = obj.ACALL
	call.To.Type = obj.TYPE_BRANCH
	morestack := "runtime.morestack"
	switch {
	case c.cursym.CFunc():
		morestack = "runtime.morestackc"
	case !c.cursym.Func().Text.From.Sym.NeedCtxt():
		morestack = "runtime.morestack_noctxt"
	}
	call.To.Sym = c.ctxt.Lookup(morestack)

	// Spill the register args that could be clobbered by the
	// morestack code
	p = c.cursym.Func().SpillRegisterArgs(p, c.newprog)

	// JAL	runtime.morestack(SB)
	p = obj.Appendp(p, c.newprog)

	p.As = AJAL
	p.To.Type = obj.TYPE_BRANCH
	if c.cursym.CFunc() {
		p.To.Sym = c.ctxt.Lookup("runtime.morestackc")
	} else if !c.cursym.Func().Text.From.Sym.NeedCtxt() {

		pPreempt.To.SetTarget(p)
	}

	// BL	runtime.morestack(SB)
	p = obj.Appendp(p, c.newprog)

	p.As = ABL
	p.To.Type = obj.TYPE_BRANCH
	if c.cursym.CFunc() {
		p.To.Sym = c.ctxt.Lookup("runtime.morestackc")
	} else if !c.cursym.Func().Text.From.Sym.NeedCtxt() {
		p.To.Sym = c.ctxt.Lookup("runtime.morestack_noctxt")
	} else {
		p.To.Sym = c.ctxt.Lookup("runtime.morestack")
	}

	}

	// BL	runtime.morestack(SB)
	call := obj.Appendp(debug, c.newprog)
	call.As = ABL
	call.To.Type = obj.TYPE_BRANCH
	morestack := "runtime.morestack"
	switch {
	case c.cursym.CFunc():
		morestack = "runtime.morestackc"
	case !c.cursym.Func().Text.From.Sym.NeedCtxt():
		morestack = "runtime.morestack_noctxt"
	}
	call.To.Sym = c.ctxt.Lookup(morestack)

	// JAL	runtime.morestack(SB)
	p = obj.Appendp(p, c.newprog)

	p.As = AJAL
	p.To.Type = obj.TYPE_BRANCH
	if c.cursym.CFunc() {
		p.To.Sym = c.ctxt.Lookup("runtime.morestackc")
	} else if !c.cursym.Func().Text.From.Sym.NeedCtxt() {
		p.To.Sym = c.ctxt.Lookup("runtime.morestack_noctxt")
	} else {
		p.To.Sym = c.ctxt.Lookup("runtime.morestack")
	}
	p.Mark |= BRANCH

	var morestacksym *obj.LSym
	if c.cursym.CFunc() {
		morestacksym = c.ctxt.Lookup("runtime.morestackc")
	} else if !c.cursym.Func().Text.From.Sym.NeedCtxt() {
		morestacksym = c.ctxt.Lookup("runtime.morestack_noctxt")
	} else {
		morestacksym = c.ctxt.Lookup("runtime.morestack")
	}

	if NeedTOCpointer(c.ctxt) {
		// In PPC64 PIC code, R2 is used as TOC pointer derived from R12

	call.As = obj.ACALL
	call.To.Type = obj.TYPE_BRANCH
	call.To.Name = obj.NAME_EXTERN
	morestack := "runtime.morestack"
	switch {
	case cursym.CFunc():
		morestack = "runtime.morestackc"
	case !cursym.Func().Text.From.Sym.NeedCtxt():
		morestack = "runtime.morestack_noctxt"
	}
	call.To.Sym = ctxt.Lookup(morestack)
	// When compiling 386 code for dynamic linking, the call needs to be adjusted

	// stackguard1 is the stack pointer compared in the //go:systemstack stack growth prologue.
	// It is stack.lo+StackGuard on g0 and gsignal stacks.
	// It is ~0 on other goroutine stacks, to trigger a call to morestackc (and crash).
	stack       stack   // offset known to runtime/cgo
	stackguard0 uintptr // offset known to liblink
	stackguard1 uintptr // offset known to liblink

	// call to maymorestack *before* processing resume points so
	// we can construct a resume point after maymorestack for
	// morestack to resume at.
	var pMorestack = s.Func().Text
	if needMoreStack && ctxt.Flag_maymorestack != "" {
		p := pMorestack

		// Save REGCTXT on the stack.
		const tempFrame = 8
		p = appendp(p, AGet, regAddr(REG_SP))
		p = appendp(p, AI32Const, constAddr(tempFrame))